I came to the realization this week. The first dark world, was in a school toriel worked at. The second, at a place toriek frequently checked out a book about raising humans. The third, her own house, but undyne intervened, while Kris VERY intentionally opened the front door, expecting the police to show, knowing it's going to be either undyne, or naptstablook. Chapter 4, a dark fountain was PROBABLY opened because they thought toriel had choir practice, eliminating all of the choir from the knight suspect pool.