I guess it depends on the age of the house. The OP doesn't specify how old the house is but there have been stories of people either storing things or items falling behind walls from attic rafters. I've heard many times that people have retrieved very old newspapers from walls. Used for insulation in very old houses. It could be entirely possible that a renovation put up an inner wall with modern insulation as to not disturb the original wall of some Victorian house. Not saying the OP lives in a Victorian mansion but some of the houses are protected from remodeling from a preservation aspect. I've seen it where entire fireplaces were boarded up and walled off, the owners never knowing it was there until a renovation.