This has been happening to my MY13 volt for about 6-8 months now. Occasionally I will get the warning ding, then the airbag light will illuminate for a few minutes, then go out again. It used to happen infrequently, but is becoming more common as time goes on. Finally went in to get it serviced today, and was quoted >$1000 for the repair. The dealership blamed it on the seat getting wet (which has never happened) or me placing my 10lb laptop backpack on the seat all the time and breaking it. He also explained why the repair is so expensive. They have to replace the entire seat cushion since the sensor is embedded in the seat, and then there is a substantial calibration process to make sure the sensor triggers at the proper weights.