You just brought back memories of the Subaru I had that thought my backpack was a passenger. I'd throw it on the passenger seat after class sometimes without thinking about it. Then about 10 minutes down the road, it would shift and wind up triggering the sensor. So I'd be having to reach and finagle the seat belt to shut off the distracting beeping while navigating a busy stretch of road where everyone likes to drive like idiots. Very safe. Luckily the previous owners of my current vehicle were kind enough to disconnect the seat belt chime and a few other safety features that I sure haven't missed. And yes, I always wear my seat belt and so do my passengers.
Ah, the old days of buckling up back packs and fish tanks, and pretty much anything else I didn't want to throw in the back that weighed over 30 lbs.