Mysterious self-moving driver-side mirror
 

The driver's side mirror on my 2001 C320 mysteriously moves when the car is parked and shutdown. Doesn't happen every time, but when it does, it is not in its normal driving position. If I push the button that restores seats, steering wheel, and mirror position, it always goes back to the proper place....This is not the passenger side mirror which moves when in reverse!

The dealer cannot figure it out! Anyone seen this or have any suggestions?