I also caught him in the act of a not so new discovery. This also happens in the bathroom. Yet another reason to close the door behind you.
The door stop is a springy one. He loves flicking it and watching it wiggle as well as the noise it makes. Our master bathroom is the only door in our house with that type of door stop. He has located the springy door stops in all of the homes we go to regularly. I am not really sure how he found ours, but he did. If we visit you, just know that he will be on the look out.