Her first thought, obviously, is sabotage of some kind. This makes perfect sense, except for how she can’t think of anyone motivated to get rid of her who has enough resources to pull this off and somehow can’t just kill her. 

Natural phenomena, probably, although she’ll have to go track down everyone involved in planning this excursion and figure out if anyone knew about it.

Her Mage Armor’s still up, so too much time can’t have passed. Teleportation of some kind, then.

She isn’t stupid enough to wander into a hall filled with, at best, smoke, or more like, supernatural darkness of some kind. The other option it is! She’ll be slow and careful, though. Even if nobody’s been through here, the rock might not be stable, or some matter of wildlife might object.

She… probably won’t have to worry about water, but she’ll keep an eye out for anything potentially edible. She has a few days of rations, but who knows where she is.

...It's getting darker.

Not like the way magical darkness normally works, just - she thinks she can't quite see so well or so far, it's a subtle thing, for now. To say that this shouldn't happen to drow is an understatement. 

This passage ends suddenly in a steep drop like a chimney, but not quite too suddenly to see it coming. She could squeeze down it fairly easily. There's a very faint sound like trickling water coming from it. 

No. No it should not.

She stops as soon as she notices anything, quietly and expressionlessly panicking. Is she… going blind? Was she poisoned? Is this some odd disease? A curse? She’ll… Detect Poison the surroundings, that’s an easy thing to check, at least.

Maybe she’s… on the verge of passing out from bad air?? She does some mental math, tries balancing on one foot. Those seem to work fine.

If she retreats her path and returns to where she arrived, does the effect end?

The surrounding air is very slightly poisonous, not in a way she can recognise, but a bit like what you see if you Detect Poison a glass of wine, or saltwater. Not enough to do this. 

The effect, if that is what it is, doesn't end if she returns, but it doesn't get any worse either. 

The smoke or fog is gone from the other passageway: she can see now that it narrows to a crawl-space, but a fairly wide one. There's an irregular clicking sound, and a draught.