You're being laughed at by a ghost, Dib. How does that make you feel?

"It's not stupid! C'mon! Who else could have done this?!"

The query, however rhetorical, implies a search. Therefore Dib's subconscious hunt for a culprit widens his tunnel vision until he actually notices the aides in nice white shirts standing near them.

"...Whoa wait, who are YOU guys?"