Here's another signal mirror video that was posted recently. Note that the user tried aiming the mirror with and without the "fireball" aiming aid, and found the aiming aid worked much better for them. Interesting to hear the reactions of folks who hadn't seen a signal mirror in action before.

(The Peter Kummerfeldt video at the start of this thread is offline at the moment, but I contacted the reporter and he says he'll get it back online - I'll post again when that happens.)

While I've posted it elsewhere, here's a WWII training video for mirrors aimed by the "double-sided mirror" method, with general discussion of signal mirror range, etc. (Slow start, but there is narrative and video after a minute or so).

Here again (not new) is my video of a 3"x5" glass mirror at 0.7, 11.1 and 43 miles (the last on a _very_clear day).

This last is the 2"x3" American Medical Kits "Rescue Flash" mirror designed by Doug Ritter, at 22 miles. Note that the telephoto video is more sensitive than the naked eye, but you can here me call out the flashes that were visible to my (middle aged) naked eyes.
A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)