Every year I hang a bell over bedroom doors. I attached dental floss to ours and used that sticky putty to secure the floss to door jamb.
Now, if he gets up in the night (to leave the room) - ring-a-ding-ding.
Most nights I sleep with one eye open but when I’m really tired, he has left the room without my knowing.
The larger bells “sleep” on the front entry door.