We live in a rented student house (in a student, dh isn't), so can not add a cat flap. We have a small back yard, a back door, behind that a metal gate that we can not remove. So we couldn't put a cat flap in anyway. Also leaving a window open all night isn't really an option either.
If our cat is out when we are heading to bed, we always let him in beforehand. But the problem is some nights if he is out (he wants to go just to be clear), when we want him in he isn't around. Once we spend nearly an hour waiting for him to appear because I refused to leave him out. Chasing him up and down the road at gone 1am.
WIBU to leave him out all night if he's not around when it's bedtime (we go to bed late as it is so can't really make bedtime any later). It's not often he's not here, just now and then.