Here's a question that's been rolling around in my brain that I'll throw out to hear your opinions.
First, I KNOW this is NOT likely...
But I've been wondering about what type of credible reason Mollie's family and LE may have (besides wishful thinking) to believe Mollie is alive. What if (again, small chance) at some point in the days that followed her disappearance, someone even momentarily turned her phone back on and it synced with her FitBit.
This would open up the possibility that Mollie had left with an acquaintance who may be holding her (think obsessive, fixating, wanting to keep her for himself), but is not treating her violently. It could also open the possibility that Mollie (at least early on) was a willing participant...going out for the night, taking off for a day or two, etc.
Again, I think she's been abducted or manipulated into leaving by an acquaintance. I don't think it's LIKELY the phone or Fitbit have given off data since her disappeance, but wondering if you see any reason to think it's possible?