Subtle effects go a long way to building atmosphere. A brief hiss of fog when opening a sealed room or object can do wonders for the mood. It may even be enough to scare off would-be interlopers from checking out something they should check out.
And you can stuff the interior with traps and deadly stuff, and then if the players complain you can say the fog was fair warning!
Commentary by memnarch (who has not seen the movie)
I.... guess this might make more sense in the movie? I think that going inside a normally closed off area would be reasonable for hiding from people, but there's still the initial problem of "why is this here" in the first place. And why does Rey look like she knows enough about it to go inside anyway? This isn't some broken down and hollowed out Star Destroyer on a planet; it's a mostly new gigantic military installation that probably still has tons of toxic vapors sealed up in places from the initial build!
Personally, I'm not sure I would want to go inside any formerly pressurized area. Especially if a fog of.... something, comes out and the ambient air temperature was pleasantly warm to begin with. And that makes me wonder if life support systems are actually mentioned in Star Wars at all. There's the force field that has a blast door close after it's destroyed of course, but nothing else seems to bother with providing breathable air.
Rey: Anything else interesting on this wall?
GM: Pill shaped light panels, coffin shapes that could be hatches, a handled lever of some sort—
Rey: The lever. Can I reach it?
GM: You’ll need to traverse one of the coffins.
Rey: Hmm. A strange lever partway up a wall with no easy access and no indication of what it does. Death-inspired imagery...
Rey: I climb across and pull the lever. 17 for Climbing!
GM: A coffin shape slides out from the wall with a hiss of eerie fog, revealing a crawlspace behind.
Rey: I climb in and close the hatch behind me.
General Hux: That seems unnecessarily bold.
Rey: Hey, if I get turned into a vampire, it’s all upside as far as I’m concerned.