I did start to wonder if the reconstruction showed that shot because they wanted to portray "Suzy's colleagues looking for her at the house" and thought that shot best accomplished that -- and/or filming them going inside and searching would mean the owner having the inside of his house on TV, plus if you only have time for one or two shots then perhaps they thought the outside one better tells the story? It's annoying though, because it really isn't clear if MG went inside -- it is very ambiguous from the AS narrative too.
Yes, HR seems to claim he was alerted by the sound of the door closing or something, if he is right either Suzy was inside and left or MG went inside and left... or he was just wrong.
I have to say, if I went to search for a missing colleague in a house like that and could not get inside -- I would not be content with just knocking on the door -- would you? You would worry that she had come to some grief inside and could not answer or get to the door, you would surely want to get inside and check she was not lying unconscious or worse). Looking through ground floor windows and a letterbox would hardly cut it. If I were worried enough to report someone missing to the police, I would get a locksmith to open the blooming door first if I didn't have keys myself!