"How did you even get up there--"

Which is what Childermass is in the midst of asking Papyrus as he makes his way up the stairs, but he stops halfway, the movement below, the sudden change in temperature, grabs his attention back down to the other floor.

"Ah." So, there it is. An open door. "That did it..."

Sorry, Papyrus, he's reversing direction back down. Feel free to follow.