Since the 60's ultrasonic detection has been used in big stores when no one is inside. I know because I could hear it, they'd leave the sound on all the time at the Darien Bradlees. I got the manager to turn it off once.
It works by sending out LOUD ultrasonic waves and reading the interference pattern that returns. Any change causes alarm. The sound frequency determines how small a move will be detected.
Who will sit completely motionless in a bathroom stall for how long?
Put a regular ping sensor in the ceiling and get empty stall distance recorded. If someone is in the stall with a fuzzy hat or soft hair the sensor will read max range/timeout for no ping return. If they make a return, the distance will be shorter than the floor or seat it usually bounces off.