Methods for detection of late night escapades?

You will need: 2 pieces of scotch tape, one long hair.

Tape a hair over the doorway to the child's room, tape a hair on every windowsill, tape a hair on any and every doorway out of the house.

You will know if someone left, the hair will only be broken by a door opening or a window opening.

It also works well for things like drawers and is virtually invisible unless you know where you put it.