She'll admit, the knock does confuse her. Jack never bothers knocking (unless the door was locked) and it didn't sound like Eddy or Jules' normal knocks...and had it been her Dad, Kim, or the Stepmonster it would have been followed by them calling inside. So instead of ignoring it, she's on her feet and opening the door.

And the moment she sees him she knows something is terribly terribly wrong. "Eddy...oh god...what is it?"