Because all Sanses are more than a little prone to embarrassment where strong motion is concerned, Sans-Serif is suddenly, piercingly grateful that he's in the lead all of a sudden. That way, hopefully Frisk can't see the overjoyed smile that breaks out over his skull at their words. The skeleton doesn't know what a fistpump is. But he's seized with the urge to do one, anyway.

"you, too, frisk." He's very, very glad everyone stopped him from hurting them.

Once Frisk gets the kitchen door open, Sans-Serif moves without hesitation. There's a pile of barrels and sacks against one wall, haphazard enough that the gap might not be immediately apparent. However, he's taken this path enough times to know where to look. He hops up onto one barrel near the wall, and then drops down onto a small segment of empty floor on the other side, between that and another barrel. There on the floor is a trap door that, with a bit of hauling, he's able to heave up and over with a loud clatter of wood and creak of hinges. Colder air wafts up from below.