Whizzy,
Whenever a girl is over at my place, I leave the decision up to her as to whether she wants to spend the night or not. If I absolutely didn't want to spend the night with her, I'd make sure we ended up at her place. That way I can decide to leave at my discretion. So since I'm not kicking her out, it's her decision to leave and face the possibility of someone recognizing her leaving my place that late. So any resulting repercussions are from her decision, not mine.
What are the actual chances of someone recognizing her leaving your place? Unless you're in a dorm, the chances of it are highly unlikely. If you are in a dorm, then I would refer you back to my original statement of it being her decision to leave at that hour.
-John