I recall this situation the OP states, happening to me and my former F10 535iA....No ASS, but we stopped at a parkinglot of a highway restaurant to drink some stuff. I turned the radio off, apparently forgot to push the start/stop button , the car is so silent. We got out of the car, I locked it. Keyfob in my jacket with me.
We went for coffee for about half an hour or so.
I came back, unlocked it, and noticed the car was still running. I felt like an idiot, with ASS I would feel less than an idiot I guess.
This is the price 'we' pay for having a startstop button/ASS and forgetting to really turn the engine off like in the 'old days' with the key.