Sans goes smoothly, perfectly still at the sight of this unknown person in Papyrus's house. His eyes flick to the nearest photographs--confirming that this is someone new in the house, and not an intruder--but otherwise Sans barely even seems to be breathing.

Then he shoves his hands into the pockets of his hoodie and grins, casual, like this is a completely normal situation. "Didn't know my brother was sharing the house now."

No point hiding that particular piece of information; Sans called for his brother when he opened the door, and anyway anyone paying attention could figure it out soon enough.