You had just spared Undyne and went to Hotland. You walked in Alphys's lab to see.... Sans and Alphys... making conversation? They were mostly talking about science shit, like timelines and stuff. They noticed you walk through the door because the door is loud.

Sans: "sup kid."

Alphys: "Oh god! I wasn't expecting you so soon! I haven't showered, I'm b-barely dressed-"

Sans (confused): "then why am i here?"

Alphys (still somewhat panicky): "That's different!"