It's a virtual cache where you give information from the grave to the CO. I think they're there to cover up said info.
Actually in this case, that is how this one was. It is actually how people have chosen to salute the man, who must have had a good sense of humor.
This is not the stone that was the virtual, just one in the vicinity. Another was adorned with a shot glass and a mushroom, another had a belt on it. Others had seashells.