My guess is the twins told their father about the gestalt experiement, and Calvin's hardware (eye-fi and the monocle...advanced eye-fi maybe?) have identified ASCII as non-biological. At this point, especially if the twins gave the account in detail, or had some recording of it, he is pretty sure of who ASCII is, all jokes aside. From there, it is a simple jump of logic to assume that the perpetrator is here to either apologize or give an explanation. Or maybe something else, but that will be seen.
Edit: Then again, ASCII mentioned this being about Calvin's work specifically.
"What the?! Where did you get THAT?!"