How old is the kitten, and is (s)he long-haired? Young kittens will occasionally play with poop or step in it and track it out of the box. Sometimes a piece gets caught in their hair, especially if it's long, and drops off somewhere outside the box.
Thanks. He's a 4 month old Maine Coon. It's possible that it did get caught up.. I don't think he's playing with it as we've had a serious discussion about hygiene issues.