Sorry to hear this. My own car (not the M3) was keyed once parked on the street of downtown and it wasn't even any eye catching ride. Some people just don't ever deserve owning nice things so they come out to damage others'.
Worse I have seen was back in high school. An open top NSX parked in a small shopping mall nearby and a group of kids went by and pouring their drinks(god knows what those are) into the car. That gave me the idea to never have the top down when parked if I ever own a convertible.
Must stop eating..............