We also spoke with the police back then (you first call them if you come and your car is gone) and they told us, yes, he does that often. The car is then parked in a locked area and you can only get it back if you pay the bill.
It stands and falls with the sign "Privatparkplatz, unberechtigte Fahrzeuge werden kostenpflichtig entfernt" - then you have no chance anymore. Of course, you must be informed beforehand that this is a private parking lot and costs will be incurred if your unlawfully parked vehicle is removed from there.