I'm thinking of hacking up something simple to check toilet occupancy in an office. Ideally, the system should be unobtrusive and out of the way.

I know you can use a PIR sensor to track moving objects (What sensor to choose to track human presence indoors?) - however, if somebody is sitting still on a cubicle, that will probably not register since they're stationary.

Apparently parking spot sensors use both IR and magnetic fields to detect cars (http://www.nedap.com/business-units/identification-systems/parking-detection-makes-sense/). Obviously the second part of that won't work on people.

I know you can get those IR-based sensors that bounce a beam off a reflector, however, I'm hoping to be able to mount just one sensor on the ceiling, if possible.

Is there another type of IR or ultra-sound sensors that might work here?

Has anybody got any particular parts or approaches they could suggest? Any caveats I should be aware of for this use case?

I'm thinking of hacking up something simple to check toilet occupancy in an office. Ideally, the system should be unobtrusive and out of the way.

I know you can use a PIR sensor to track moving objects (What sensor to choose to track human presence indoors?) - however, if somebody is sitting still on a cubicle, that will probably not register since they're stationary.

Apparently parking spot sensors use both IR and magnetic fields to detect cars (http://www.nedap.com/business-units/identification-systems/parking-detection-makes-sense/). Obviously the second part of that won't work on people.

I know you can get those IR-based sensors that bounce a beam off a reflector, however, I'm hoping to be able to mount just one sensor on the ceiling, if possible.

Is there another type of IR or ultra-sound sensors that might work here?

Has anybody got any particular parts or approaches they could suggest? Any caveats I should be aware of for this use case?