He was only 200 meters from home and it took them twenty six years to find him? I just looked up what a "sheep fold" was and it's just a simple sheep pen.
Most of the people who lived in the same building as Dahmer had no idea he was killing, dismembering, and even eating his victims in the same building as them.
Josef Fritzl managed to hide his daughter in a secret room in his basement, with his wife convinced she had ran away. She gave birth down there six times and her mom, just above her head, was completely unaware.
He went missing in the middle of a civil war when lots of people were being kidnapped and/or killed. Maybe because of this they didn't look as hard as they otherwise would.