Front Air Bag Sensor Control problems of the 2011 Mazda MAZDA3

My 2011 Mazda 3 driver seat has come off track while I was on the highway. I went to slide my seat back and the seat popped off the track, luckily I caught myself. Mazda dealership said they wouldnt cover it. Now, 3 weeks after my warranty ended, my passenger airbag sensor has lit up, on the dash and above the radio. Mazda said it was my sensor under my seat but still had to charge $105 dollar fee to find the problem??? the airbag sensor came on one morning on my way to work. Nobody was in the car except me.