Leaving the phone behind is the main reason that my theory is some kind of abduction. Someone orders her to toss the gun down and then orders her off the roof, is in control of the situation.
Otherwise, to go from being on the phone (and using it frequently) to leaving it on the roof is really concerning. At this point, I guess anything is possible but she was relatively coherent less than an hour before she disappeared, so I don't think she ran off into the woods on a dark night without her phone. Dogs would not do much if Ada's scent was in and around the cabin, she had been in and out of her own car, it would be hard for a dog to unwind all that. A perpetrator would not want her to take the phone along.
If she were flagrantly psychotic, she could have hallucinated a reason to run off without her phone into the woods, but at her age, such an event would (IMO) be unlikely (although if drugs are involved, I guess all bets are off).
I wonder where she kept her extra ammo.