Eeek, I never followed up on this thread. A couple of years ago I did raise the passenger seat to the highest position so I could get a good look underneath while everything was still connected. I found several (I don't recall if it was 3 or 4) micro-switches near the seat track. I used Caig DeOxit followed by Caig PreservIt to clean and lubricate the switches. That seemed to do the trick, no further instances of an occupied passenger seat that wasn't.
-Rod