There was a post at least a couple threads ago that I thought had some good ideas and I forget who posted it. But the person had mentioned rural mail carriers (or people like mail carriers in general) knowing the ins and outs of who lives where, who is gone, who normally has cars at home, etc. I think this is an interesting line of thought to consider. We have seen it happen before with a delivery person who came back to their route to attack.
If it's not someone MT knew well, I could see this being a really good possibility, even more so than a totally random outsider. Someone who is in the community or maybe lives in a nearby community but has regular business here. Someone like a delivery person or mail person (note, I am NOT accusing MT's mail person!) that would notice the comings and goings of residents, when cars that are normally at home are not, that kind of thing. Someone who may have seen Mollie when they were doing whatever their business is but may have never even spoken to her or just in passing "hi" kind of a thing. I am assuming LE has already questioned folks like the regular mail person, delivery drivers, etc that would have a reason to frequent the BF's house and her running route, but just a thought.