Seems like the tree would have grown around it or at least knocked it over as it grew.
I'm thinking maybe it hasn't been there near as long as the age of the rifle.
Don't know how long it has been there but I guess it isn't to hard to lose one. I showed a person where to hunt turkeys and never thought to much of it, couple of days later my friend and his wife showed up at my door. Seems he shot a turkey and leaned shotgun against a tree to get bird. Went to pick it up and turkey flopped around on the ground. Every time he tried to get It it got further away. Finally it got up and ran away. This is when he realized he didn't know where he was and where he put his shotgun. he was to embarrassed to come to me and tell me. His wife made him come up. Drove back to the spot, walked down in the woods and there was his weapon. He finally told me it took over four hours to walk back out because he was lost. He found a hunter that showed him which direction to go. I think he would have left it there if it wasn't for his wife. He was lucky we didn't have any rain or it would have looked like a rust bucket.
I teach that every time a gun is touched, it should be checked for its status - loaded or clear. Even if you watch someone check, you should double check. If the cop in the article and video Ric-san mentions had done so, there would be no story. The fact that he didn't check shows he was in the wrong too.