Ive before seen guys hide their keys, only to have a short while latter a different guy access the car. So i stopped him and it started to get a little heated. He eventually told me the his name and who owned the car, then after proceeded to get out a wallet and prove that. It turned out okay and he settled down and acutely was rather apologetic and was really happy to have people looking out. But I've never questioned any one every again..You simply don't know

I take a key into the water, its the only safe way IMHO..
Also a side point is in South Africa thats how Shark Spotters program got started. A guy paid a local to sit and watch his car whilst surfing. He later took two guys one to watch cars and one to watch the water

Maybe we could use dole bludgers or work for the dole to watch the car parks