As we were taxiing on the tarmac to the terminal, the overhead speakers announced,
12B. Your seatbelt appears to have slipped off.
How did they know? Conductive fibers in the seatbelt webbing to create a circuit? Strain gauges on the seatbelt terminations? Lucky guess? Or did 12B actually start to stand while the plane was still in motion?
I don’t know. Do you?
Update. I took another flight on the same airline a week later. Wouldn’t you know, the passenger next to me released his seatbelt immediately on landing. There was no announcement. I have to assume the offender on the first flight let his seat belt fall into the aisle.1