The two times it has happened today was when I was climbing up a stairwell in order to get away from some mob that had seen me and was still chasing me. So I was testing something of a gamebreaking thought (that I could run away by going up dungeon levels), something like other rogue-likes.
OH! This was in the main game mode. I believe it happened w/ the class where you can knock out ppl who don't know you're there, and the class where the first person who sees you is seduced to open doors. I know it was on "green" levels too if that happens to matter.
I'm glad that my words inspire you :D -- it's really a good idea and well executed in design.