It's probably a security issue to reveal the locations and types, but my technical side is really curious. I'd imagine that they are all quite ingeniously disguised, as GenerationX pointed out, as I also have never really noticed any obvious ones.
I know that most rides have IR cameras, and I have heard that there are IR cameras scattered throughout the park.
With today's technology a camera can be smaller then the end of a pencil lead and put just about anywhere.
Just food for thought...