Heated seat fire

I got to work this morning at 4 am. At about 1 pm a coworker told me my car was on fire. I ran out to find my whole car filled with smoke. The drivers seat had caught fire and was spreading. We were able to put it out but some damage was done. The seat, carpet, door panel, and sun visors are goners and the top had minor damage. I assume the seat heater shorted since that's where the fire started. Has anyone heard of this happening? The car was obviously not on so I don't know why the seat had power to it.