My POV/Theory: At this point, I believe the investigators have a decent idea who is responsible and currently gathering solid evidence for a conviction. Everything indicates that she made it home that evening and either went missing late July 18 or on the morning of July 19. (Per Fitbit info as reported by the Jacks' neighbor, DC, and Mollie's computer activity as reported by JT as relayed to him via LE). The Jacks found their dogs in the basement, and per DJ and BJ, they are always released once someone is home. So, one can reasonably assume that Mollie released them when she returned from jogging prior to working on her school paper. With the dogs being found in the basement after Mollie went missing, and the house not disturbed, then it's likely that she confined them to the basement before she again exited the house (likely to work) in the morning, especially with assumption her red work shirt is missing (although that's in debate). She didn't pre-arrange to retrieve the shared car from her brother, despite an important event the next day at work. Not many would wait until the morning of work to figure out transportation when one needs to arrive at 8:00 AM (time, per co-worker). That would suggest that she had already pre-arranged a ride with someone else. With her digital footprint being kept under wraps, I think her cell phone and computer info has already led LE to a suspect/person of interest related to that possible ride arrangement.
