It happens more than you would think. I hate to admit how many guns I’ve found at the gun club I belong to.
I was one of the range officers at our local gun club and often checked the ranges a couple times a day as I was driving by. One one stop there was this fancy Weatherby bolt just sitting on the bench and no signs of anyone around it. I took it in my care and ran it down the local shop for the dealer there (another member of the club) to lock in his vault. Found the owner the next day as he was trying to find me. He was a non-member there as a guest with a rifle he borrowed from his FIL. When it recoiled the scope caught him to the point that he couldn’t really see and had blood flowing pretty well. The guy he was there with packed him off to the ER and it wasn’t until they got home that they realized the gun had been left behind. He was really really grateful to the point of a dinner for me and the wife. I take it that his FIL valued the gun much higher than a son-in-law. 
