Given almost everyone hiking has a cellphone with them, nowadays...
Interestingly, the news is reporting that the canyon searchers have scanners for bluetooth radio, where I looked up some of the details.
*Local authorities will use drone to scan for Bluetooth frequencies in park*
*How a scenic enclave at the bottom of the Grand Canyon turned into a epicenter of deadly destruction*
This article explains what they "can" use to find survivors or equipment.
*Drone-mounted signal detection gives rescuers a faster way*
*to find missing persons*
Apparently in the Grand Canyon, Bluetooth works well due to the active beacon, and even if a smartphone battery is "dead", the BT beacon may still be working, and BT is also in earbuds, smartwatches, medical sensors, and, of course, luggage/wallet tags.
Many bluetooth devices can't easily be turned off, so they'd still be active, while cellular and Wi-Fi, by contrast, are probably turned off.
And dead people, trapped in a sudden flood, don't have time in general, to activate satellite transceivers, which is why BT scanning is used most.
They're also checking license plates, apparently.
*Rangers checked license plates across park to find unaccounted-for people after floods*