2016/06/12 - "Initial search-and-rescue responders were able to locate the victimís husband because he had brought a signal mirror on the hike."
Injured Hiker Rescued near Hailey

Hiker dislocated knee during fall in location w/o cell coverage. Her husband hiked to location with coverage, signaled with mirror, provided GPS coordinates. Site was Kelly Mountain, west of Hailey, Idaho.

Only other coverage I've managed to find:
Technical rescue for injured hiker on steep terrain
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A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)