I hate to give spoilers, but in this case it’s terribly obscure:
after you see the ‘you’re being watched’ tag, read the bookcase mentioning the Odd Keystone then immediately and without moving press Start and open a menu that obscures the overworld screen.

Well, Lucki, you heard the internet stranger. Go obscure your vision in the creepy abandoned place where someone is watching you.

… Lucki observes, staring at the weird giant rock thing that definitely wasn’t behind her a second ago.

Shahhh! it says despite possessing no vocal cords, lungs, or indeed, movable parts of any kind.

Lucki is attacked! By a L50 spiritomb that promptly suicides what the fuck. Tirtouga celebrates by evolving.

They can live both in the ocean and on land. A slap from one of them is enough to open a hole in the bottom of a tanker.

You’re right, pokedex, it does seem like tempting fate to base your power source on the ability to industrially murder things like this.

Right, let’s try that again with masterballs on hand this time.

I save right beforehand and decide to see if it happens again. Yup, first move.

I guess it is legitimately capturable, you just need another pokemon with memento and that move that blocks a pokemon from using any move yours does, but still really tricky.

A Pokemon that was formed by 108 spirits. It is bound to a fissure in an odd keystone.

Which is not capitalized, because when has any of this made sense. I guess you find Odd Keystone and make friends and then that attracts a spiritomb in its own regular non-sapient odd keystone.

Okay, let’s go see if there’s anything else in the letter room, like another card on the chair.

It’s a bed that seems to have been chewed on. There’s something underneath it…

It’s a colorful magazine of some kind. “The Fame and the Shame! Champions Exposed!”

No Lucki, put that back! You’re too young to be holding someone’s ancient pornmag. A girl your age should only be seeing wholesome fanfic porn and not naked images.

And wait…why two beds? The Cozmo apology was in the other room, as if that’s the guy’s room, but this is the room with his son’s letters and desk covered in his kid’s toys. So maybe the other room was some other guy’s, and he had the apology because Cozmo had to hand that out to everyone, maybe, and the spiritomb happened to show up there just because I was reading about it, and the unemployment thing was just part of the company folding. Lot of explanation there.

Further on, some kid hands me a key. They claim to have no idea what it is, but the narration informs me it’s the key to room one, so presumably there’s an obvious 1 or something on it.

Hey, other kid, do you have a key too?

I heard this place used to collect something called e-n-e-r-g-y.

… Uh, translation issues? The only thing I could think of this meaning is if the Japanese version had them using a different language for the word to hide what it was/sound brand-namey.

Anyway, so – it seems like this place is distinct from Devon since they’re working on it relatively recently, so were they a competitor who’d decided to get energy by destroying ecosystems rather than individuals? But it seems like tempting fate to be drinking Devon energy drinks in that case, since people okay with pikachu furnaces would probably be fine with a little poisoning, so maybe it’s not pikachu furnaces but wailmer ones and this is just regular genocide energy by Devon? Whales are historically a great energy source.

Off to Room One! The key sticks in the lock and the door clearly hasn’t been opened in a long time, but somehow there’s a trainer pair inside! And by somehow, I mean they are the unquiet dead.

Stop it. Don’t get too close to her!

OKAY SURE YES ANYTHING SPOOKY GHOST GIRL ANYTHING

Don’t come any closer! My big sister is all mine!

OKAY SURE UNDEAD LITTLE GIRL

My babbled attempts at appeasement do nothing and they attack me with a chingling and kirlia, so it’s time for Plan B, screaming “kill kill kill” and hope my pokemon can fight them off.

Scall: Hey, are you OK, Sis?!
Ion: …Scall…

I decide to risk talking to them afterward.

I must admit that you have power, hisses the larger undead. But I must warn you… You mustn’t get too close to her.

Wholly acceptable terms! I thank you for your mercy. Smaller undead?

I don’t hate strong Trainers, so we could battle again sometime.

I appreciate your lack of animosity, terrible spirit.

My big sister is all mine, though! You can’t have her, no matter how many times you come for her!

OKAY YES TOTALLY FINE NOT A REAPER OR ANYTHING WITH OPINIONS ON THAT MATTER HAVE FUN IN WHATEVER AFTERLIFE YOU CHOOSE DON’T KILL ME PLEASE

also in the room is a key. i take it and flee.

i wonder how they died. i wonder if their bodies are in the two dented lockers that can’t be examined.

Moving on to the second room, which hopefully won’t be full of the jealous dead.

AAAAAAAAAAAAAHHHHHHHHHHHHHHHHHHHHH THIS TIME THEY LOOK LIKE NORMAL PEOPLE OH GOD IF GHOSTS CAN LOOK NORMAL HOW MANY HAVE I MET WITHOUT REALIZING IT

I decide to edge around them and check out the surrounding area first.

The file labeled “CONFIDENTIAL” contains stained documents. The documents have dates way before you were born.

[]De– Sec– Investi– –port
The develop– on new –rgy turned out to be true. The ene– that uses –kemo– bio–rgy is called –inity Ener–.
[]Investi– Report on Wa–
A series of actions related to the cancellation of the –w Mauvil– Proje– tur– out to be true. I recom– prompt discip– action against him as a traitor to our group.
Do–
Team E Leader, –ative Group —

Huh.

Okay, so this is Devon, and I guess this is where they figured out how to do genocide energy? But Steven’s dad is old, and he said it was his grandfather that figured this out. How old is this place?

Maybe the first report is from the very beginning. I don’t think we know how long this place was running.

There’s a crumpled piece of paper inside. “Sea Mauville Company Song” is written on it.
1. The daybreak shines on Route 108.
Wake up, our towering fort.
For the bright future Hoenn,
We won’t take our days off.
Dig through, dig out great energy.
Ah, Sea Mauville, Sea Mauville, Sea Mauville.
2. The bright light shines on Route 108.
Intensely shiny, our fort.
Swear your selfless devotion.
Unionization is out of the question.
Dig through, dig out great energy.
Ah, Sea Mauville, Sea Mauville, Sea Mauville.”
3. Darkness descends on Route 108.
Tonight again sleepless, our fort.
Drive away our competitors.
We won’t take our overtime pay.
Dig through, dig out great energy.
Ah, Sea Mauville, Sea Mauville, Sea Mauville.

What.

Also oh, I get it, spiritomb and it’s on route 108. Everything about this place is ill-omened, huh?

There’s an old note at the back of the cabinet with broken glass.
“Finally, the date of closing has been decided. I can’t say thank you enough to Stern, who invited us to his shipyard when we would have been left adrift. Dock, who’s done a dirty job, and I can both start with a clean slate. The only way we can repay him is to support his business with our skills.”

So is that supposed to be dramatic irony, or are we going on the idea the guy who runs Devon’s shipyard and in the present day is constructing a genocide sub is less evil than whatever Sea Mauville was doing?

…well, time to get the attention of the ghosts.

…Blush…

Uh…that’s all you have to say? Well, I’ll take it over being murdered by ghosts.

Wha– How in the world did you get in? I locked the door…

Other people might take this to mean there’s a totally reasonable explanation for why these girls were in a locked room that doesn’t involve them dying back when this place was functional and haunting the rooms. I call those other people “distractions” because they’re still standing there while I’m most of the way through the door. The narration makes it clear the locking mechanism on these doors is ancient and no longer really works.

She did lock the door, but that was many decades ago.

Hm. Given Japanese often uses family terms even when referring to non-family, there’s a good chance the other set of undead weren’t actual siblings but unrelated extremely protective and possessive girls. Maybe I can identify the ghosts by who’s lesbian or possessing of the subtext. I’m not sure why that would be – maybe ghosts are worse at hiding their feelings? Maybe Hoenn’s become even more intensely homophobic than the time of hinting, so even this sort of thing is too risky? That’s super depressing.

All right. You want this, don’t you? Take this!

Now I have the key to the sixth room. I can’t tell what this means. Is she partly aware of the situation, enough to know that to accomplish her standard wish (GTFO) she needs to offer what the modern explorer wants, rather than the fellow employee or boss of her time? Or is this the key to the stockroom or something?

I was teaching this girl how hard it is to be a Trainer!

Seems legit. Who doesn’t lock doors in a room with no wild pokemon encounters and blush a lot while learning how to be a trainer?

Which raises a more intriguing point than that there are gay ghosts here – this is supposed to be a pokemon habitat, but the inside is pokemon free. Did they coat the walls in arsenic or something? But there’s some few plants growing, and pokemon are even more diverse than plants (plus, even the pokemon plants need less photosynthesis than regular ones and you’d think sun would be in short supply down here). Why can’t a single one manage to find a niche here? What did they do?

To the sixth room to see if there’s any answers there.

It’s a tattered clipping from an old magazine cover. On the cover is a young lady who looks like Lisia.
“Termination of New Mauville Project Officially Announced at Last!
Serious Talks Begin about the Closure of Sea Mauville!
Cries of Concern Erupt from Staff Members Working at Sea Mauville!
Greater Mauville Holdings Reps Remain Tight-Lipped about the Issues!
Project Leader Wattson Weighs In on Problems Plaguing New Mauville!”
The rest of the magazine has been torn away…

So…not closed because genocide is wrong, or at least not officially.

Is that Lisia’s mother or grandmother? I’m confused about the timeline here, but I’m pretty sure it can’t be her. Anyway, if she’s even tangentially aware of what was actually going on here this gives her a further motivation for why she’s trying to make a new way of interacting with pokemon.

It’s an old, hide-bound notebook.
“The damage caused by the cancellation of the New Mauville project has been catastrophic. As a member of the management, much of the blame and the debts will fall upon me. But that will be little consolation to the employees working under me who will lose their livelihoods. I couldn’t do a thing for them… I’m a man with no power… Protecting nature and Pokémon and the environment… It’s great as an idea. It’s a fine ideal to aspire to. And Wattson is a great man for dreaming of it all. But cruel reality and the organization that I must try to preserve have dashed those dreams. I couldn’t do a thing for him… I’m a man with no power…” This is the last page with writing on it. Other pages have been torn out.

Huh.

Well, this certainly highlights how very separate but equal pokemon are. They’re just a nebulous good like “nature” and “the environment”.

If this place was running pikachu furnaces, isn’t this, “Yes, yes, untold horrors, but think of what this does to the concentration camp guard’s job security?”

Also…can’t people get new jobs? The pokeworld even seems to have pretty good social services.

There’s a opening to the outside as well, and a door below which seems to need a storage key. Also, some guy fishing with no luck but who did somehow dredge up the fourth key in his attempts. There next, I guess?

It’s a card that has mold growing on it.
“Sea Mauville Employee ID
Group Leader Stern
Excavation Unit No. 8 (Team I)

So yeah, confirmation Stern’s evil.

There are dirty papers inside the cabinet…
“Sea Mau–nal Newsletter #389
“We ha– a new winne–he monthly slogan–mpetition! It’s–alty–the lea–Storage Unit No. 1– ith– Don’t dirty the seas with pollution and dumping! Dirty your uniform with sweat and hard work! In recog–of his win, Cap’n Sal–will–ceive a Rocky–met and–yperior! –everyone will kee–slogan in the–hearts and–as they giv–re all again–s month!”

Interesting. See, the obvious would be that they were trying to keep their image good here, but I think it’s more sinister than that – polluting would reduce the pokemon around, and they need those for the furnaces. It makes me think of this K-drama episode I saw where this company graciously installed hand sanitizers in the poor districts that were actually transmitting plague so that they could sell the cure for it to the richer people who got infected secondhand.

Back to the storage shed. I enter.

Hey, would ya look at that? Somebody got it open! spiky-shouts someone offscreen who somehow noticed this. And indeed, it’s that thug guy.

Well, if ain’t that kid from before.

Think you accidentally a word there.

Looks like you done good, huh? Youse the one who opened this?

Lucki, damn her, just lets this guy run by. No, Lucki! Have I not been clear? We kill all who oppose us, dammit.

Wh-what in the world’s this all? They’s Nuggets over there! And they’s Nuggets over here! This whole joint’s packed with Nuggets, ain’t it! No wonder the capo was lookin’ for this place. What a punch in the gob!

Good idea there.

Good work here, kid. Now scram.

Right, so now attack?

No? Lucki apparently stands there for whatever timelapse it takes for some other guy to show up.

…You found it? says the old dude.

Mr. Raizoh! Couse I did, boss. We hit the jackpot!

He ignores this and heads over to the one non-itemball item. Ah… My Takao… So this is where you’ve been… You must have been so lonely here… I’m sorry… Come on… It’s time to go home… And he picks up what I assume is a skitty doll and leaves in a cloud of mental illness.

H… Hold up! Uncle?! Thug-guy starts to leave, then stops at Lucki. Hey, kid! Looks like this is your lucky day! Looks like all this shine is yours.

You realize in the time it takes you to tell me this in your rush out the door, you could’ve instead grabbed at least half and then rushed out without explanation?

Also I’m disappointed, I was expecting him to say I’d better not touch it because he’d be back and then I could wonder what would happen if I actually did leave it alone and if it’d disappear later.

Keep your nose clean!

So then I get a million nuggets. Why is this place packed with gold chunks? It sounded like it was the prize room, and a company that doesn’t want employees taking their overtime pay doesn’t seem like the sort to give out prizes that are functionally just cash. There’s also a beedrill mega stone here.

Well, I didn’t find another key, so I guess that leaves only going deeper.

And…nothing down here but spookiness.

Time to mash A?

There is something glimmering faintly in the water… Will you pick it up?

No, let’s think about this.

…huh, still can’t see it. Okay, grab it then. But since it seems like it’s probably Devon technology, that means it’s packed with murdered souls, so let’s hoof it now before any of the cursedness of it rubs off.

This seems like an actually hard puzzle to solve if you’ve just played the new games. The item detector doesn’t work underwater, but for most of the game you haven’t needed to try to guess where things might be or to search for them like this, you just used the glow antenna. There’s a slight hint in that there’s other underwater items you can find, but I don’t think I’d have found those if I hadn’t already known how to find stuff from previous games.

Anyway, back up to pass the vengeful ghosts on to that scientist.

Ahhh! Is that the Scanner? Hmm… Do you think you could deliver it to Captain Stern?

Some people might say I’m doing the bigger favor giving it to you so you can take it back and pretend you actually did your job.

I still have things I want to inspect while I’m here.

Yes, but I don’t want to deal with Stern now that he’s double confirmed as pro-genocide.

Hey! Who said you could just turn away and ignore me!

Okay fine I’ll go give it to Stern and explain how he should fire your useless ass. To Slateport!

Oh, how are you doing? The other day, I bumped into a guy who worked under me at Sea Mauville. He seemed pretty successful in his career now, but he was still apologizing for a mistake he’d made in the past. He said something like, “I’m terribly sorry that I lost the Odd Keystone.” I don’t remember it at all, but it made me feel nostalgic and happy.

Well, aren’t you a piece of shit.

Okay, so Stern isn’t at the shipyard. Museum? Nope. Love their music, though, makes me feel nostalgic in a way that doesn’t have anything to do with pokemon mass murder!

Ho! That device you have there is the Scanner from Sea Mauville! That could make our expeditions really take off!

You know, even without the full confession, this should be a red flag. If people know how to make it, why is it they need to dredge up an old one instead of making a new one? Unless the way they made it involved something they might get in trouble for these days.

Would you do me a favor and let me take that Scanner off your hands?

The curse targeting you would be karmicly appropriate, always a good thing with ghost curses, but I don’t like how you jump from “a favor” to “take it off your hands”, like you’re trying to convince me it’s basically worthless and you’re sort of doing me the favor. Plus, I can always drop it at your feet or something, so I lose nothing by trying to bargain with you, and you’re a jerk who deserves to be taken advantage of.

You drive a hard bargain. Let me know if you reconsider.

Again with the suspicious jump. Not giving it for free is a hard bargain?

But he’s right to hold off. I don’t want the damn thing near me. Fine, take it, asshole.

Yes! Thanks. This will help our research a lot!

Is your research finding new pokemon for the furnaces?

Take this bell as a token of my appreciation for your generosity.

Clear bell. Hmmmmmmmm…..

Also still gonna kill you, genocidy asshole.

That bell figures in old legends from the Johto region. It’s pretty nice to have.

So Devon’s also into looting other cultures for the shinies?

When I was working on Sea Mauville, I had a lad working under me who was from those parts, and he gave me that.

Sure he did. Some random kid from those parts had a sacred item while working on mass murder, and he handed it to his boss for no reason.

If I remember rightly, his family passed down some special dance style. But it was only for the girls, so as a lad he came here looking for work…

Hm. So some disgruntled son in the family, angry at tradition, steals their sacred object and then decides to work for the kill pokemon for electricity company. Okay, that’s now seeming more plausible.

He always kept that bell on his person, but sometimes when we were up top there at Sea Mauville that bell would ring, and it would almost seem to glow. It sure was mysterious, all right.

So, animated by Suicine’s anger at the despoilment of nature? But the corporation was too dangerous for even a legendary to take on?

I’ve held onto it all these years, but I’d be happy if it could serve you some use now.

Huh. So while I’m trying to pass the scanner and its ghosty vengeance off on him, maybe he’s trying to redirect a seething legendary at me.

Well. That…that sure was a thing. And a thing that involved very little of people telling me exactly what to do, and flavor text everywhere!

I wonder if it’s because it’s a wholly optional area? If someone finds the concept of going back and forth between rooms too taxing, it doesn’t stop them from finishing the main game, so it doesn’t matter of some hypothetical kid can’t manage it.