Back when people were really worried, about the US and Russia slugging it out, they used to build bunkers underground next to their houses and connect them with hidden tunnels or doors. You go in the house you wouldn’t have a clue there was a bunker next to it. The entrance would look like a normal wall with shelves or just paneling. These can be quite elaborate with multiple floors. Sometimes the old owners die and the new ones never know its there? I’ve even heard of some with firing ranges. And the best part …the tax assessor doesn’t know.