A: No; and B: Yes.
Longer answers: I’ve never, ever, taken a signaling mirror on my trips but on a recent 6-day trip (July 2011) in the Spanish Sierra Nevada I blamed myself for not carrying one. One of my friends had gone missing. Last time we saw him, he was ahead of the rest of the group going up a pretty large mountain and, although we had arranged to meet down in the valley, we followed him up that same mountain, thinking he would wait for us. Well he didn’t. To make a long story short.... When he realized he had made a mistake, he went down into the valley, only to find out we weren’t there (we were still looking for him high up). To get a really good idea about the situation, here’s some pictures:
Pic 1: Shows the valley “Lavaderos de la Reina” (the red arrow points to where my friend was - almost invisible with the naked eye).
Pic 2: Same picture with a 16x zoom.
Pic 3. Again digitally zoomed in a little bit more.
Pay attention to the black dot at the right of the red arrow in pic 1. That’s the cascade in pic 2. In the centre of pic 2 there is a small stream and 2 rocks: a white one on the left and a dark one on the right (the white one isn’t visible in pic 1 and the dark one hardly). Well.... my friend was waiting exactly in between those two rocks: yes next to the stream a bit nearer to the white rock. To be more precise, those tiny little black dots (hardly visible on pic 2) is himself, his pack and his dog. I prepared pic 3 (on the computer) to get a better idea.
Why did I upload these images? To make it clear my friend was really far away from us; we only spotted him whilst he was on the move. The fact there wasn’t anybody else and that his dog was with him, made us think it was him. I used my 16x zoom camera to find out it really was him. Well, made the picture and then zoomed in digitally; only then I could tell, more or less, it was him.
So now what? Shouting was no good; waving didn’t help. Mobile phone??? No network available :(. It was at that moment when I remembered some survival classes I took over 35 years ago where I was trained how to use a signaling mirror. But...... I didn’t have one. I was watching the picture I had just made of my friend when, all of a sudden, I realized I could use the back of the camera to try to signal him. I tried it out on a nearby rock and indeed I could see the reflection. I moved towards my friend and...... INSTANT CONTACT. He waved, stood up, picked up his pack and walked towards us. We did the same and about 40 - 45 minutes later we rejoined (this means we were at least 3 miles apart -maybe 4-).
This anecdote taught me many things. It’s easy to say: We’ll meet at point X (in this case, my friend had been there before so I thought he’d have no problem in getting there), but it’s even easier that something crops up that makes all the planning not working out well, so...... never, ever allow anyone to get separated if you’re out in a group. The strongest (quickest) person should NOT lead the group; he should stay at the back (to check that nobody gets behind) and the leader should guide the group (to where ever you want to go), making sure he doesn’t walk too fast.
It also taught me, and this is the real answer to Jim’s second question, that you can use the back of a camera as a signaling device and it works VERY WELL.