A coffin might have to be moved or there are no family members alive anymore - oh any number of reasons. Hence no need for a gravemarker anymore. We might believe our final resting place will be for eternity but often it is not.

What I mean is the graveyard caretakers has to do this but I dont think they are more prone to haunting than any other person. I am half a skeptic to the existance of ghosts.

Sometimes grave markers are removed and destroyed though by the caretakers for several reason. Its a bit upsetting to see them.

I am not really scared of them but certainly respectful when I visit one.

My mother usually says there is no need to be afraid in a graveyard because everyone there is already dead. I think she means living humans are what you should be afraid of.

I saw a documentary once where they were talking about how after WWII and the liberation of the concentration camps, when they were demolishing them - locals would come in and take the building materials (bricks and such) and build houses out of them.

"authorities noted that locals had dismantled most of the remaining camp buildings, reusing parts of them in their own houses."

Edit : Sobibor