Undertale AU?

Spoilers ahead for True Lab and Pacifist Route and Gaster.

Gaster's wife (and Sans and Papyrus' mom) fell down/got really hurt, and was near death. In a moment of desperation, Gaster drained DT from some of the human souls with the already existing DT extractor. She survived, but was semi-melted. They both agreed it would be best for her to remain hidden in the lab. When he fell into the Core and disappeared (read: was scattered throughout the time-space continuum), she kept away from the main and True labs, staying in the rarely used storage and misc. rooms. Frisk and Alphys see her when checking through the rooms for Endogeny, who went looking for something (there are no cameras down there, so they can't just go through the feeds).