In my experience (7 years of NSW State Emergency Service)
(a) People are very often on the trail. An initial report of "lost" typically means "didn't report in on time". Many times that means that they're just slow, or injured, or walking in the wrong direction on a poorly marked trail.
They get found, no one makes a big deal of it, it never gets on the news, so it doesn't register in our consciousnesses, but it's fairly frequent.
(b) The first step in a co-ordinated search effort is to search the paths. If the search area is large (e.g. we know where they parked their car, but not which path they took) then you can need to arrange a fair number of searchers to cover all the paths. That's slow. Not just to do the searching, but also to get a team mobilised and coordinated. If drones can do that job (even just a first pass) it saves a lot of time and frees up a lot of searchers.
(c) There's "trails" and then there's trails. In Australian bush, a "trail" might not actually be created by humans. It could easily be a kangaroo path, and walkers sometimes stumble on to them and end up in places the searchers didn't expect. The drones might be better at searching those paths.
(d) It's not uncommon for someone who gets lost to eventually stumble upon a trail and not know where it is, where it leads, or which way to go. Just because they're on a trail now, doesn't mean it's the one they started on.
(e) For people who want to be found, there's likely to be a fairly high success rate in drones that move along all discoverable trails, emitting out emergency messages and listening for responses. Most human search teams aren't doing much more than that.
I guess my perspective is skewed. I am on a mountain sar team. The sheriff handles most of the "never gets on the news" cases, and calls us when it is going to the news.
Lost often does include people on trails. Lost simple means they did not return, which might include they died or where injured on the trail, are lost of a trail, change they plans and didn't let someone know, etc. Beyond that, this would be very useful for not getting lost; think Google maps for trails.
Move faster for a short time. They could certainly fly down trails looking for people via image recognition, but its hard to compare than to a person walking a trail for 8/10/16 hours looking for all sorts of signs, even smells.
Many lost humans are also nowhere near trails. Old people, people with mental issues, and young children often actively hide from searchers. Flying robots might not get the results one would expect.
This project is made for an alpine area. With steep as- and descends a drone would be much faster than a human and a dog. And a thermal camera would be a good replacement for a dog. However, most people get lost while weather conditions with bad sight and a drone wouldn't help much here.
