My cousin lived next to a crazy old man and his family for most of our lives. The old man just went about his day carrying a shotgun, mind you over here its very rare to see anyone carrying any guns so boldly, and was quick to anger. His family always argued that it was an old useless shotgun he kept from being a hunter in his youth. No one knew enough about guns to argue otherwise, but it did look pretty old. Eventually he kicked the bucket and the family moved elsewhere, selling the house to a relative of theirs that my uncle knew. It was pretty sudden. So this relative is just casually drinking with my uncle and offhandedly mentions that the house was terribly built so he was getting the walls and some floors redone. He mentions that the crew found this big block of cement buried in sand in what seemed to be what was left of an underground water reservoir, joking that it was about the size of a person. My uncle asked him a few days later but he panicked and denied anything of the like.

There were always rumours that the old man had killed someone due to how quick to anger he was, and this almost solidified it in my uncle and cousin's minds.

tl;dr that my cousin's crazy neighbor killed someone and he&his family hid their body inside their house. Further supported by the family leaving as soon as he died and the new owner backtracking after mentioning finding a suspicious block of cement when remodeling the house.