I see posts on VFTT every so often for lost items. If I remember right, I've seen posts about lost cameras, snowshoes (usually just one), jackets, plus a few found items. Sometimes they get returned when people recognize the item. It works there because so many of the members hike in the same area.

If I came across a tent, I would leave it. I left mine up in a camp site in NZ while I hiked into a hut for a day or two. I think that was pretty common in that particular park. Didn't think to leave a note, but not a bad idea.

Sometimes you see stories about lost cameras where someone posts the pictures on Facebook and after a while someone recognizes someone in the pictures. A while back there was a website in NYC with pictures of lost gloves and mittens, not sure if it is still up.