My thoughts from what I've read so far as a cyclist and someone who lives in a rural Colorado community relatively near Monarch Pass:
1) If she was a SM user, I would be very surprised if she didn't have tracking apps active and wearables logging her location on all connected devices. I'm not the only one who regularly rides with Strava, Map My Ride, and/or Apple Workout all tracking at the same time. If she had a wearable like a watch vs a bike-attached tracker like a Garmin, chances are one of those apps would have continued tracking her movement....
4) I went MTBing by myself about three weeks ago and txt'd a local friend where I was going to ride and to notify LE if I wasn't back by 5pm. Why her instead of family? Well, she's local, if I needed help I could call her, I can also tell her the recreation area and trails and assure she will know the general area and be able to communicate said to LE. I then went boondocking for a weekend by myself and told my neighbor that I'd be back on Sunday. Why? Well, they've got eyes on my house my family doesn't. They know the area I said I was going to and could notify and communicate that to LE better than family could. Both of these are routine events and I'm not calling my family every day when I hit a trail or the road or go skiing or ......., but I'll be with people or people will know where I am.