Deduction is a great tool for players to apply to unusual situations.
It's great for GMs too when players use deduction, only they refer to it as "overthinking".
Commentary by memnarch (who has not seen the movie)
Or (d) there are nocturnal or cave dwelling beings that have a preference for the dim and dark and this is the part of the bar where they hang out. Or (e) the owners don't care about what the non-patron areas look like so they saved money on buying equipment. Or (f) any combination or all of the above.... you get the idea. Lots of different reasons for having a place that isn't made specifically for the PCs. I don't think most players will be thinking about these sorts of things, so they'd make for a good twist on the usual expectations.
In-movie, this feels like quite the jump in atmosphere from Finn getting found out as the traitor, and I'm not sure what to think of that. Maybe there's a scene cut that's not a part of the comic? Rey doesn't seem to be moving that quickly or cautiously as evidenced by the walking down the middle of this hallway with the bright overhead lights. And BB-8's following not exactly close behind probably making a racket thunking down the stairs.
Commentary by Keybounce (who has not seen the movie)
[Keybounce's comments will appear here when received.]
GM: You reach a gloomy hallway, the ceiling heavily braced with stone archwork. Lamps on the walls barely pierce the tenebrous murk.
Rey: There’s something very fishy going on here.
BB-8: Why’s that?
Rey: You could easily light this hallway using brighter bulbs. Either (a) the darkness is what some would consider to be unnatural...
Rey: Or (b) someone just didn’t want people down here to see very well. Which makes me ask, “Why?”
BB-8: Saving power?
Rey: But the lights were on when we got here. If you wanted to save power, you’d switch them off.
BB-8: So... they want patches of dim light and shade. There’s probably a monster lurking in the shadows.
Rey: Almost certainly. Excellent deductive reasoning.
BB-8: Or... (c) he just wanted an excuse to use the word “tenebrous”.