The subtle smile happens to me when looking, and when driving.
As I pulled away from a red light a few weeks ago, my daughter riding shotgun said "Daddy, was that massive acceleration really necessary?!" Grinning ear to ear, I replied "No; but it was fun!"

 

Same, here. Usually look back at it 2-3 times after parking. Same thing happened with my M3 for the first couple years though..