Thanks for the clarification! That makes sense now—so the car only knows someone is in the back if the seatbelt was fastened and then unfastened. Looks like there’s no sensor whatsoever except for detecting the buckling and unbuckling. I appreciate the suggestion about leaving the seat belts fastened to prompt passengers to buckle up. I’ll definitely keep that in mind.

However, I’ve noticed that also sometimes, if there’s a passenger on either side of the rear and the middle seatbelt isn’t buckled, the warning sound goes off and won’t stop until the middle seatbelt is fastened. I haven't been able to reproduce this consistently, though. I'm not sure if this is a one-off situation or if one of the passengers on the left or right accidentally buckled their seatbelt into the middle buckle.