You could probably glue a flat magnet to the end of deadbolt and place a reed switch inside the box. Though you'd have to be careful to never apply force to the

You could probably glue a flat magnet to the end of deadbolt and place a reed switch inside the box. Though you'd have to be careful to never apply force to the door as you're unlatching the deadbolt to avoid detaching the magnet. And you'd need to run wires to the reed switch, which means drilling holes in the door frame.