Thanks for all the thoughtful comments.
I can confirm the car was in P - I checked the screen because I thought maybe it somehow was stuck in H. But definately P - the map was still displayed - it looked ready to drive off in - which is why I was concerned. No-one in the back and no pets in there. Was totally empty - I couldn't spot a phone or a card key.
The photo doesn't show just how close it was to the wall, I saw the driver and passenger park as I drove in, and vaguely recall seeing the driver too close to open his door, so I honestly think he climbed out the passenger door (there was enough room and he had a passenger to help).
I was surprised Tesla actually answered the phone (I called the service center) and then was suprised they could lock it - I didn't actually ask them to lock it, I asked them to contact the owner, but a minute after I hung up the call I saw the car lock and lights turn off! Nice to know Tesla can actually do that!