Sounds like it was in Park and took up play on the parking pawl
Yes, that is exactly what it felt like but I never take the car out of D unless I'm going to be sat for ages.
I can remember where on my journey home it used to do it (at the time, there were roadworks going on so always traffic jams).
But then again, if no one else had this in their Es or CLSs, I must have imagined it...