When I used to live in a house I had one of those motion sensor things and it often got tripped at 2-3 am which would waken the woman I was living with at the time (don't ask me how, maybe she slept with one eye open). We never spotted anything, until one night when we were up late for some reason and she was looking out the window and saw our neighbor's daughter crossing through the yard (low brick walls). The neighbor's daughter was 13 at the time, so we ratted her out.