Just throw yourself into an overall, find a crowded parking garage, put up a sign “leave the key in the lock switch” and tell the arriving people where to put their car, possibly blocking other cars (which is ok, as you are there to “move the car” if need may be). Nobody will suspect anything.

I guess these arrangements for squeezing out every drop of capacity of a parking garage are fairly common, but I was still surprised that I left my car in the hands of someone without asking for any kind of proof/id. It was just so convincing. A small doubt only came into my mind when I had long left the garage. Of course, in this case it was not a scam and nothing happened. Still funny how in some circumstances one is super alert (… holding on to your wallet while walking down La Rambla …) but then one doesn’t even think twice before giving away one’s car.

Needless to say that if you put on a police uniform you could do much crazier stuff …