Hey Etch.... I had the same problem on my 2012 audi a6 just after purchasing the vehicle (bought it used). You are completely correct by saying that it is every time someone sits on the Centre seat in the back seat. There is a control unit underneath the seat which causes this issue every time people sit. The wiring to the unit goes loose and causes a shot, which results in all the many faults on the dash.
Please take the car in and have your auto electrician properly reconnect the wires to the unit and you will be sorted.
