Every one of us has some secrets that other should not know of. In cases we need to hide something, we also need some hiding places that nobody can find. Therefore these hiding places could offer you the secret space you are looking for.
In a standard house, there can be various hiding places. Furnitures, decoration objects, materials used in building, ceilings and any possible place. When you look for a hiding place, you should be careful thinking about the possibility for others to find that place. If you think nobody can find that place, then it is good hiding place.