- Who else indeed- Santa-capped busybody
If you follow the path of Diane's sneaking
and where "the busybody" was standing, Diane must have made physical contact with the spot where he was without seeing or touching him.
Granted, there's an entryway to a small kitchen area where he could have been beforehand, so it's *not* physically impossible. It just would have been really difficult to get in that spot in any sort of normal fashion without Diane noticing it, even if just in her peripheral vision.
As for the conclusion that Nanase has reached... Next comic!