I don't know anything about skip tracing and haven't really been following the Brian Laundrie/Gabby story. I do know a little about the AT. Yes it's like 2K+ miles long but it's like a great big small town. People know each other and keep track of each other, they see things and pass gossip up and down the trail.
There's a lot of people involved with the trail. There are the thru hikers and day hikers. Some hikers have hiked it more than once. Scouts working on the trail or walking it in troops. Various trail clubs that maintain sections of the trail and shelters along the trail. BTDT. There are or were non-profit search and rescue groups that specialize in wilderness rescues. There are park rangers who patrol, find lost hikers and help keep the peace. There's folk who provide trail magic, yes that's a thing. I've run into military from the UK on the trail. In and around Camp David? Hikers may or may not see our military. I guarantee you they're there. Property owners along the AT (not all of the trail is on government land) keep an eye on who comes and goes.
I'm not mentioning moonshiners, wild mushroom hunters, weed growers, poachers, Ginseng growers/poachers etc. No not going to mention them.
This isn't 1995. A lot of the trail follow ridge lines. Yes there are dead spots, lots of them. but in this day of cell phones? Cell service can be a short uphill hike away or available on the other side of the ridge.
Hiding on the AT? Might be easier said than done. Doing it long term without outside support? Is it possible his family hid caches along the trail and sent him the GPS coordinates? Possible. Still not seeing hiding out on the AT as a viable option.