I recall in one of the early press conferences, the officer saying something to the effect of Mollie doing something "out of character" and "Getting to know Mollie better" or something to that effect. It made me think that maybe she'd either been romantically involved with somebody other than her BF or possibly been active on a datng site such as Tinder.
In either case I could see a jilted love interest being very upset about her leaving to the Dominican Republic in a few days on a romantic wedding getaway with her BF and contacting her while the BF and roommates were out of town to discuss. She agrees to talk, gets in a car with this person and *poof*...gone
I have been thinking this way as well. It stands in contrast to the narrative about the hog farmer, who is highly unlikely to be someone Mollie would have conversed with on social media.