Wow that really blows. I really don't understand some people these days. Probably bikers that stole the stuff too, which says something about that persons integrity and dedication to a sport that they are involved in.
I also had this same thing happen recently. I left some shovels and a rake at a local trail spot. I thought they would be safe, seeing as how they are being used to create the trails that people ride on everyday. But no, while I was out of commission with a broken thumb they were stolen. I guess this is one reason to have "secret" trails that no one knows about it, but where is the fun in that?
It really sucks when you can't even trust some of the people in your own sport...