Maybe she had opened the door carefully, just a crack at first (a flashback to the first season when she stood on Cusamono's toilet and spied on his house) and then opened it up with no pretense after she saw him tear the page. She was already looking for any provocation or reason to dump him, a way to blame it on him, and so I can see her approaching the door on that occasion in a deliberate fashion. Correct me if I'm wrong, but I think he was a bit restless because she was a few minutes late letting him in, which could have been another act on her part to get him on edge so that she could absolve herself of responsibility for the ambush she was about to launch and make it look like it all suddenly came from him.
In any case, that's the sort of detail I tend to be quite forgiving on. I think we're just supposed to accept that she had an instant to see what he was doing before Tony (or we) were able to know she did.
Tony, his spirits crushed after b-lining to the fridge first thing in the morning: "Who ate the last piece of cake?"