There are a number of things that cross my mind, and wonder about.
Since Mollie was dog sitting, I would expect there to be no one else in the home except for her, no brothers of her boyfriend or anyone else, so neither of them could be a suspect nor able to provide any information about what time she got home or went out.
Since Mollie was dog sitting I suppose she went for a walk with the dogs later that evening?
Since the police and boyfriend are optimistic she is still alive, then probably there were no signs of a struggle in the house, the door was locked with the key, and the dogs were taken care of the proper way someone would prior to leaving? Or did she take the dogs with her?
If something would happen the dogs would get upset, would be heard, would be found by themselves without food and water, or also be missing.
Since there is no information about the dogs, and the search for the 21th was called of, some information probably surfaced, and some answer is in the dogs situation. IMO