I stumbled upon a couple in Maryland once, they were right on the trail, ski trail that is, there was about a foot of snow at the time, and they had been xc skiing. I chose to bushwhack to avoid surprising them, but after walking for hours through the snow up and down hills without ever finding the cache. In hindsight I probably should have waited and stayed on the trail. It didn't offend me at all though, we were probably the only three in the woods that day, so no harm done.
In light of the British past time of "dogging", I'm surprised there aren't event caches dedicated to this spectator sport! Or are there some out there that I just don't know about?