I just experienced this as well. I found a thread which suggested some possible causes, one of which was something blocking the front or back of the seat. I had thrown a coat behind the passenger seat on the floor, when I put it on the back seat the airbag turned back on. Sometimes a car can be too smart.
My first week with our new M3 RWD. We go to dinner tonight and I see the same thing. With my wife in the passenger seat it says the airbag is off. Then she shifts around and it goes on. Then it goes off agin. She is right around 100 lbs. Surely seat sensor can't require over 100 pounds to activate the passenger air bag. And I can't ask my wife to sit in the back seat forever. Did anyone find a remedy for this? I have a 14 Oct appointment for Service for some delivery issues. I'm going to add this to the list.
It is indeed right around 100lbs according to Tesla. And keep in mind the sensor isn’t exactly accurate as a scale and parts of your body weight are not on the seat (eg lower legs) so if she weighs 100 lbs there won’t be 100 lbs on the seat cushion.
No need for her to sit in the back, it’s just for someone of her size the airbag might do more harm than good.
The sensor on my 22 M3 is a large flat plastic part sitting on the springs below the foam of the seat bottom.
I have modified a lot of seats. I had one seat act up. I never figured out what was wrong but what I noticed that there's a wire that runs parallel to the heater wire in the passenger seat only. That wire is a single loop. It is sewn next to the heater wire which is underneath the faux leather cover on a stock OEM seat. That makes me think it's some kind of "ground plane". My guess is the sensor can determine the compression of the seat foam by measuring the distance from the seat bottom cover wires to the plastic sensor under the foam. Given the sensor is supposed to be "on" for over 100lbs and a person weighs very close to 100 lbs, the sensor could go either way. The manner she sits, how much she leans back into the upper section, all may have influence.
The driver's side is completely different. If something presses the seat almost at all, it is "on".
The sensor is extremely simple flat pressure switch glued to the top of the foam, under the faux leather cover.
So you wouldn't have these issues in the driver's seat.
I can't say which is safer for a person very close to 100 lbs to have the Airbags on or off. It seems it was designed to be on at 100 lbs. Can you do a service under warranty?