An Idea
So Tammy had somehow given a basic idea of what she wanted without even realizing it. Her daughter was quick to take notes and tried her best not to disappoint her mother who had given her such a comfortable life growing up. Even though Tammy was not expecting something from her kids, she was about to realize that she was going to get something nice really soon.

Giving Away Just A Small Clue
Even when someone plans a surprise for us, many times we often find ourselves getting a little suspicious or we find something weird in their behavior. So just like this, there was a clue of sorts that Oswalt received when Sammy was about to surprise her. She felt that something was a little off because Hewett pulled out her phone and she turned on her camera.
