Re: Trying to trigger sensor, you could also buckle a seatbelt assuming they aren't already buckled (you don't mention year on your X, but older ones had LATCH system in third row while I don't believe newer ones do). Some sections of some Teslas definitely treat latched seatbelts as a form of occupancy detection.
ETA: My 2017 X definitely has occupancy detection in the third row, as it will tell me a seatbelt is uncovered when my buckled kid puts his weight on his hand in the empty seat next to him. I wouldn't think they could remove this feature, as I would expect it to be required regulatorily. As such, it seems most likely that a software bug is the culprit if buckled seatbelts aren't triggering occupancy detection (though it could also be a stupid design choice/oversight at some point in time/software, as Tesla definitely does stupid things with their software intermittently).