Not sure if this counts as lost but I had about a dozen guns stolen from my mother's house while I was in basic training back in 2007. Even managed to get two of them back, one of them just last year.
I've also "found" three guns in my life.
First was a 16 gauge single shot that was in the branches of a cedar tree in the woods of my grandparent's farm when I was about 14 or 15 years old, so 2002 or 3. Turns out it was my great-great grandfathers, had his name carved into the barrel under the forearm, and was lost by my grandfather's oldest brother while he was out squirrel hunting and had to make one of those urgent stops. He was out hunting while home on leave after getting back from Korea in 1952 when he lost it. Was in surprisingly good shape considering it had been there for 50 years and my grandfather let me keep it and practice some of the gunsmithing he was teaching me with it. After about two years of finding or making parts for it, I was able to get it back to shooting condition and even killed a few squirrel with it. Still have it sitting in the safe now.
Second one was a nickel plated S&W model 10 that I found laying on a hiking trail a few years later. That one I picked up, put in my pack, and kept on hiking. Ran into a guy rushing down the trail about a mile later who asked if I had found a gun on the trail. Told him I had and asked him to describe it and when he did I gave it back.
The last one was about ten years ago and I didn't so much find it as I was just there when the owner abandoned it. We were both at a local WMA range at opposite ends of the line shooting. He received a phone call then urgently packed up and left quickly, only he forgot the second rifle case laying on the bench. I went down and picked up the rifle and, when he didn't come back before I was finished, left a note that basically said "if you lost something here call and tell me what it was and I'll give it back" with my phone number stapled to the pole next to the bench he was using. Two days later he called me and asked if I had his rifle and arranged to meet to get it back. Turns out the call was a state trooper calling from his wife's phone telling him that his wife and daughter were in a bad car accident and were both on there way to the local ER.