Just got the car back from the service shop at my local MB dealership. They were unable to re-create the problem. They checked over the transmission and parking brake.

Just to clarify, the car was on a very slight grade, just enough to get a ball rolling. The car rolled slowly about 1 to 2 feet. It was almost imperceptable. Once I realised the car was actually moving, I engaged the parking brake.

Anyway, thank you for your helpful comments, (even carnut). I will keep you posted if it happens again.