What if Frisk stayed though? Imagine an AU where Frisk stayed with Toriel and after a week or two Toriel starts to tell Sans about  them. They would talk after a few rounds of jokes about their life, Sans would talk about Papyrus and Snowdin and Toriel would talk about new recipes and now Frisk. Everything goes smoothly for a few months until Alphys looks over the video feed and overhears their conversations about the human. Undyne finds out either by accident or Alphys telling her the truth, after all, she hasn’t seen much of the human yet, and there goes the peace.

Sans is confronted by Undyne as to why he isn’t trying to do anything and even seems pretty chummy with whoever is just beyond the door when they’re hiding the last human they need. Everyone’s salvation is just beyond that door and he didn’t report anything. Well, Sans, what are you going to do?