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 him.

Or see the horrific Josef Fritzl incident, where he managed to hide his daughter in IIRC 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.