There's a retarded teen in my neighborhood who once came to my door offering to sell my wife a set of kitchen knives which had clearly been well, well used.
My wife politely declined and he must have just tossed the knives all over my lawn in a moment of frustration, because my daughter and I found them the next morning.
I found out later he'd been going door to door trying to sell them all over my neighborhood that day - I guess he just totally gave up after my wife said no.
Anyway, I went and asked his mom if she was missing any knives and she was confused - then she told me that his latest thing was looking through people's garbage to find salvageable stuff he thought he could sell.
I guess my point is, if it were a developmentally normal teen who pulled that little stunt I would have been extremely concerned, but in this kid's case I knew he was just trying to do a little business.
Thus my advice to the guy who started the thread - talk to the parents first, police second.
We had a retarded/deaf kid in our neighborhood. For some reason, he decided to hang out at our house. He was a huge kid who I was told was inappropriate with young girls. He followed my daughter and her friends (who was MUCH younger). Maybe he was just lonely. We were always nice to him. My husband even loaned him money from time to time but we finally had to dissuade him from coming over. I never knew if the accusations were true about him, but couldn't take the chance.
I still think of that kid and what happened to him. He had nowhere to go and nothing to do. Sad.