Many just detect temperature changes. And upside down can will spray cold air. If there’s any space between the doors or between the door and the frame, you can spray it in and it’ll detect the temperature motion. Fancier models do exist that are more specific, so YMMV.
Anon is on the path to finding out why it’s like that and trying it out for themselves.
Breaking into buildings by triggering their REX-sensors?
Maybe I shouldn’t ask but how do you trigger an exit button from the outside with a can of compressed air ?
Many just detect temperature changes. And upside down can will spray cold air. If there’s any space between the doors or between the door and the frame, you can spray it in and it’ll detect the temperature motion. Fancier models do exist that are more specific, so YMMV.