My theory on this has evolved but I've settled into a voluntary visit that turned into an involuntary missing, and possibly very dangerous situation. I won't or can't speculate on who she was with, but I do think some kind of relationship or encounter gone wrong. My personal opinion is that LE is sorting through her phone and electronic records to validate, confirm, and develop leads. I also think it's possible that they may not know who she went to visit until they get her phone records, OR they do know who it is, and that person is either a prominent and/or married citizen which LE has to tiptoe around and without probable cause, can not pressure or name in the press.
I have read carefully posts from those who are familiar with how LE agencies in Utah operate, and that their tight-lipped approach to this is NOT an indication that this a voluntary missing situation. We don't have true visibility into urgency or lack thereof - for all we know, LE is working fast and furious behind the scenes on any working theories and persons of interest they have. While there are a few anomalies in this, in how her family is reacting (or not), I think it's just still too early to reach a conclusion that they are leaning this way.