I mostly hike in the Virginia, Maryland, Pennsylvania area - so the trees pretty much prevent using a signal mirror unless you first climb to the top of one mountain ridges and find a cliff / overlook.
Or so I've always thought - until recently...
I went day hiking along the Potomac river during the spring thaw in February and as I was leaving the trailhead I noticed several fire / rescue vehicles pull in and a dozen SAR guys hustle over to an overlook and start looking down the valley with binoculars. When I asked what was up, they told me a hiker had been hurt and they were flying in a helicopter for evac. As I started my day hike, I noticed a helicopter slowly circling one of the overlook / rocky points jutting about 2-3 miles farther down the valley.
The dayhike trail followed a ridge that had several overlooks - and for then next 30-45 minutes I noticed the helicopter was still flying around in the same area. At approximately the 1 hour mark, I noticed they were now flying a "back and forth" search pattern. About 30-45 minutes later, their search pattern legs lasted about 5-7 minutes each way - and were long enough so I could hear the helicopter for 3-4 minutes each leg.
At about the 2 hour mark, I noticed they were now flying the same back and forth pattern - on the other side of the valley - in a bowl shaped side valley. About 45 minutes later, I noticed they had moved to a different side-valley. After another 30 minutes I noticed they had moved to a third location.
At the 3 1/2 hour mark, I noticed they were not flying the great big back and forth pattern (no sound for 3-4 minutes at a time) on the side of the valley they had just searched the 3 locations at.
Around the 4 1/2 hour mark the search pattern had moved far enough away, I couldn't hear it anymore.
Anyway, I got me thinking... What I decided was that if I was injured enough to need rescue, it would probably be near a cliff. Otherwise it would be a sprain I could wrap and walk out on. Further, I figured if my buddy had a head injury or internal problems, etc - I might not be able to leave them and hike out to bring help. Finally, if I was alone, I might be too injured to make a fire to generate smoke, or lay out clothing in a big X pattern etc.
So I think that a mirror might be a good idea. The helicopter would be able to see it, since the cliff doesn't have trees on 1 side - and the SAR guys searched those areas first. I've also read mirros can be seen by pilots a few miles away, and the flickering light gets their attention better than smoke, etc.
But what if it's not sunny the day someone's injured... For those days, I bet those fireworks smoke bombs weigh around 1/3 - 1/2 ounce each. I might start carrying a mirror and a handful (2-3 ounces) of smoke bombs just in case. It would really suck to be injured, be able to call the rescue chopper - and still not get rescued. I bet that chopper searched a 5-7 square mile area - and that area had several named trails with good landmarks to direct help to your approximate location.