As someone suggested earlier, it's possible that Sirius Black is hiding out as the Weasley owl (the "measured and courteous hoot"). That would fit with Peter Pettigrew being the unfortunate in Azkaban chanting, "I'm not serious."

It's also possible that Pettigrew is hiding out somewhere, I suppose, but that doesn't seem smart.

This also raises the possibility that someone or multiple someones who weren't ever Marauders using small animagus forms to get around the castle, which could show up funny on the map.