[For all Papyrus knows, she very well could be a fellow carnie. He doesn't know everyone's faces, and especially given the quick hiring policy the Ringmaster operates with... Well, those ears on somebody who doesn't otherwise seem to have fur seems clear enough.]

Oh, the usual reason. They're trapped, and bored, and acting out. I sympathize! Really!

[He looks up at the ferris wheel, squinting his eye sockets as if that'll help him see details that far away.]

Whoever it was did it when I wasn't looking. The only way they're getting these socks back... is going to the lost and found. And fessing up!