RE: the thief example...
Here's how I could see this going...
(Thief): Is there somewhere I could hide here? What's the cave like?
(GM): Sure, there's a rock outcropping that's not visible from the entrance, and it curves around, blocking sight from most of the cave. Plus, it's pretty dark.
(Thief): Sure, I'll slip behind it, press up close, and extinguish my torch.
(GM): Cool! Right before you put your torch out, though, you notice the wall in this alcove looks a bit different from the rest of the cave. What do you do?
(Thief): I take a closer look, bringing my torch up close. If it's safe, I try to rub the spot that's different.
[At this point, maybe a discern realities? But probably not even that, I'd probably just say.]
(GM): It looks like it's covered in dark dust, or maybe soot? It reminds you of the inside of a fireplace or furnace.
... at this point, I think sufficient information has been given for them to make a decision; if they stay here, they'll get blasted by fire, of course, but I'd give them a chance to evade. Or, maybe the ceiling is high enough that they could climb up and hang there, and they'd be outside of the breath's area, but they'd need to make a str or con roll to endure hanging up there for that long while they wait.
So, it's pretty much just part of the conversation, you show signs of danger and if they leave you a golden opportunity, you make your move...
Let me know what you think...
- Alex