Mine started doing that periodically after my daughter got out of the car when I dropped her off for school with the car still running. Ironically, it happened 75 miles before my 36k B2B warranty was about to run out, and she had an out of town track meet after school followed by a trip to Ohare airport and back which would put us a few hundred miles over. The GM service advisor took pity on me and did the $800 fix for $100 because she also had to wait 2 weeks before she could see me (some medical emergency with their volt repair guy). I have no idea how hard it is to replace the passenger presence module, but that was the fix for my car. This issue didn't happen every time my daughter exited, but probably 20-30% of the time before I had it fixed. And yes, sometimes while driving, it would pop up that service airbag system message with nobody in the seat.