I was fishing a popular spot on a popular stream in ID once upon a time when I hooked some willows on my backcast. When I went back to retrieve it, I saw the fly and reached for it, only to discover that it was not my fly, but one someone else had broken off in the same willow patch. That gave me pause to wonder about how many other flies were in those willows. So, I put my rod down and spent 5 minutes combing through those willows and came away with 8-10 more flies! They ranged from ones that looked like they had just come out of the vice to ones that were weathered and rusty.
I stick found flies in the carpet in my truck near the shift lever. There is quite a collection there at present. Occasionally I'll use one. However, in all the years I've been fly fishing, I can count on one hand, with a couple fingers left over, the times I've caught a fish with another fly already stuck in its mouth. One time it was my own fly that I had broken off in it just a half hour earlier.
D