The main issue there is flight time. Most small drones have very limited endurance.
That said SAR already use drones to help search when weather permits.
For autonomous systems one thing I find interesting is how far they have to go in this sort of domain versus a good human pilot for example:
https://youtu.be/bFVvUeP_oqY?t=36
Speaking of SAR, well the other SAR (Synthetic-aperture radar) it was discussed yesterday that a radar reflector can be used to bounce SAR signals back, which would highlight your location, such as it's used for marine applications.
Perhaps carrying one of these folded up ready to deploy in an emergency wouldn't be a terrible idea as the availability of SAR imaging goes up. Much cheaper to carry that than some of the GPS/beacon devices sold to hikers. Image detection is already a common ML tool for Earth observation analysis so building in something that looks for that marker in a known area of forest sounds possible.
The traditional version of that is a bright orange square or signal mirror. They pack down really small and are extremely visible. Similarly little IR and retroreflective glint squares are handy for spotting people at night.
That said if you're doing anything serious away from people carrying a GPS and EPIRB/SPOT is way more effective. The latter working even in the most remote locations. It's no good being able to signal if no one is even looking for you.
There is already a product on the market call RECCO. It is a two part system very similar to what you described. The reflector is seen directly into coat, boots, you name it. SAR then uses a device that sends out a signal that bounces off the reflectors.
To the best of my knowledge, all SAR drones operate above the tree canopy. The forest I live by has, even at peak occupancy, more people sized animals than people. It makes thermal imaging through a dense canopy difficult at best. A good friend spent 2 years as a coast guard pilot stationed near me, and he spent the majority of his time doing SAR training. He was the one to describe the issues with thermal imaging above the canopy. We discussed using drones at length, and his main issue was the risk that they might randomly enter his flight path and damage a rotor. Operating below the canopy would eliminate this risk.
I think some sort of hybrid balloon/drone could be more suited for certain tasks. Still, the demonstration shows that we could have a fleet of whatever navigate a forest.
That said SAR already use drones to help search when weather permits.
For autonomous systems one thing I find interesting is how far they have to go in this sort of domain versus a good human pilot for example: https://youtu.be/bFVvUeP_oqY?t=36