When Norm returned home he stood in the doorway to the kitchen wondering where Alice was when the killer attacked, however there was nowhere for the killer to hide along that wall without ever being seen. The position of the couch made it impossible for the killer to have ever been in that location.
When Janet is showering, the killer is taking photos of her through the window. At some point she hears the clicking of the camera. That would be impossible, not only due to the noise of the shower and washing her hair but also the humming she was doing and the fact that the window was closed.