From what I can see you don't even have to knock the door, there is an intercom, which if used would raise even less attention than knocking. Another thing I noticed about the door, is there is no handle (being an ex Joiner you tend to notice sad things like that lol) You have to open the door with your key and push it open by the looks of things. Even if she left the door unlocked, you wouldn't be able to get in from the outside, not until the Yale handle was turned from the flat side to move the lock bolt.
Unless someone had gloves on, which was quite possible considering the freezing weather, then if they pushed their way in, their dabs should be on that door somewhere. That is of course if she did get home. I wonder if it is at all possible to read anything from that intercom?