Look, he knows you're not in there... kind of. Then Sans catches the pink glow behind the curtain, next to the obviously-not-broken window. He'd crack wise, but he's actually never seen a ghost outside their corporeal form, so color him curious as he steps forward.

"... Wheelies? That you?"

He won't lie. The temptation to look there, but as much as he's ragged on Mettaton before... Sans Learns a Privacy. That and, well. He can put two and two together.

"Nope, it's just me. Papyrus not here either?" He tilts his head, quiet for a moment, then he adds: "... so are you getting ghost-dressed, or can I pull this back?"