Rather than ringing the doorbell when there is nobody there (and sometimes refusing to ring when there is someone there), or doing that odd humming thing he does sometimes, today he was simply sitting in the bell and whistling in an ear-piercing manner.
He could clearly hear me coming, because the first few times he stopped whistling when I stood up to track him down. But I am too sneaky to be caught in such a way. I crept up on him through the other door, caught him at it, took his cover off and gave him a good clonk. I hope this will be a lesson to him.