When Billy is worried where Allison is and everyone is standing outside discussing it, Sandy opens the door of Apt. #7 "next" to the apartment where she actually lives, Apt. #6, and you see her standing in that doorway.
When Allison finally comes clean with Billy in the kitchen and tells him that she was just helping Jake study for his GED, Billy is paused at the hallway entering the kitchen. As he converses with Alison, he steps forward and you can see the "mark" taped on the floor where his cue to stand was.