If you're willing to tinker, a camera that compares the current image with a "baseline" image and says it's occupied if there's enough change. You'd have to ha

If you're willing to tinker, a camera that compares the current image with a "baseline" image and says it's occupied if there's enough change. You'd have to have an IR or NIR camera to work at night, and take special care around things like doors/drawers/curtains that move.