In May of 2016, I'd picked up my BMW i3-REx and was driving West from Charlotte NC to Knoxville TN. It was sunset so I pulled into Biltmore Park to get summer and a quick charge. Eventually I found a free, L2 charger and plugged in. A guy sitting on a bench said, "Be careful, your car might be unplugged."
I asked "Why?" and he said just to 'mess with you.' So I locked my car which locked the J1772 to the car. An hour later, the sun had set, a good meal, and the car was still plugged in.
So far, both cars have locked the plug in when the car is locked. So I'm not too terribly worried about being unplugged with the exception of the passive J1772-to-Tesla adapter.
Bob Wilson