Well, maybe he didn't make the trip himself. Maybe his trash helpers did and got rid of her phone while they were at it.
I really want to believe that Kelsey is alive and ok, but three weeks is a long time to not contact someone from her family. Plus, her vehicles were still in her driveway, so how could she have gotten to Idaho on her own? By now, I am sure Uber, Lyft, taxi companies and bus stations have been checked for the possibility that Kelsey used one of those to get somewhere, but I doubt very much that she had enough money to pay for any of those modes of transportation (bus maybe, but Uber or Lyft or Taxi- no way). It is possible she met someone and left with him or her, who knows.
Still, my head tells me she is no longer alive, and LE is on the right track as to who may be responsible. My heart doesn't want to believe it, though.