Sensor to detect toilet occupancy?
The automatic flush systems used at airports, movie theaters, and restaurants all use active IR technology, not PIR.  Active IR uses an IR transmitter and an IR receiver. There is one sensor per toilet.



They are located behind the toilet seat (or on the top of a urinal) and bathe the immediate area with IR light.  When a person enters this field, the IR bounces back to the IR receiver and after a few seconds "arms" the unit.  When the person leaves, after a couple of seconds the control unit flushes the toilet or urinal.

Commercial units cost hundreds of dollars, but at least at one time you could pick up a complete unit for a home toilet here, supposedly for $24 (I realize you don't need the flusher part, just the sensor).

There are quite a few videos on this subject on YouTube (search for "automatic toilet flush"), including ones centered around the ubiquitous Arduino.