I'm certain this sort of thing happens often. A few years ago I was grilling out on the back yard one evening. I heard the phone in the house ringing and ran inside to answer. It was someone asking about the price to acquire use rights for several images for a CD cover. I said it depends several factors, which I briefly explained. I then asked the person if we could talk in the morning because my dinner was about to burn. The person then said "Well, actually we've already used them on the CD cover". He explained that it wasn't by accident but they were now thinking twice since the CD was software that would be purchased by churches. Oh the irony. We exchanged information, I was paid a reasonable price, and that was that.
On another earlier occasion I found my images while browsing postcards in a national park. That would normally be a good thing, except I knew nothing about these. It turned out they'd been publishing these for five years before I found them. I ended up needing an attorney for that one. In the end I got what was left after paying attorney fees. It was a hassle but satisfying to have done something about it. Perhaps it saved someone else from the same fate.