Two questions on night gaunts:

1. Is there a consistent set of rules that governs where you can and can't call them, other than that the room has to be outdoors?

2. If you try to call one in a room that's off-limits does it trigger an unambiguous "room is off limits" echo, or does it just look like the gaunt failed to capture its target?

1) Yes.
2) From what I could tell the nightgaunt just stops following you immediately - there's some guesswork as to why.