Busted! Lock him up!

We had a similar discovery once, when we were moving out. Our cats liked to play with the little plastic ring that comes off a milk jug, and we never gave much thought about what happened to them until we picked up the sofa to load it, and there were several dozen milk rings lying. Apparently, the cats would knock these thinsg around until it went to the one place they couldnt get to, and so there they stayed until we moved out.