But she didn't listen, she was in the bathroom, she was getting dressed, she was putting on make-up, she was getting ready. Getting ready. - Robert Bloch

Norman Bates heard the noise and a shock went through him. It sounded as though somebody was tapping on the windowpane. He looked up, hastily, half prepared to rise, and the book slid from his hands to his ample lap. Then he realized that the sound was merely rain. Late afternoon rain, striking the parlor window.

And then, as he ripped back the shower curtains and stared down at the hacked and twisted thing sprawled on the floor of the stall, he realized that Mother had used her keys.

Once you began speculation about that, once you admited to yourself that you didn't really know how another person's mind operated, then you came up against the ultimate admission — anything was possible.