One of the most creepy themes involving old buildings is a new owner investigating only to find a trap door, a movable bookcase, or a wall that was covered over to hide an unknown void....
All of this gets even creepier when people wonder what the room was used for or perhaps they find someone was living in that space all the time they were on the other side of the wall feeding their children, watching TV and making love. If that doesn't give one shivers, I don't know what would!
One chilling story is told by a 15-year-old. His family moved into a home that had been vacant for a year. They lived in it for a year before the boys were playing in the master bedroom, one of them bumping into a bookcase that moved. They took their smartphones and went down an iron spiral staircase to find a crawlspace. After crawling in, they found a room with wrappers from their Halloween candy and odd miscellaneous things like dolls and a key. The children scrambled out of there and police were called to investigate. It was determined that someone had been using the room at various times. The area was boarded up. (LINK)
Plenty of times homeowners have found hidden spaces with time capsules and treasures hidden away and rooms for the Underground Railroad.
The historic Civil War Hospital estate I grew up in, there was a fireplace in the dining room angled oddly. The family who had been living there when the war broke out were Quakers. When my curious siblings and I investigated, we drilled a hole into the hallway floor above and then into the wall and held a light to the hole in the wall and looked through the hole in the floor above and saw a space with a stool, some cloth, a newspaper of some kind.... We could only assume someone had made a hiding place for runaway slaves, perhaps making some loose bricks in the fireplace to allow entry and exit when needed.
Celebrities are not immune to this issue. Jennifer Lopez had a male stalker who managed to live in her pool guest house for a full week, even posting pics from there.
In China, a man noticing he kept losing money and food items and other things in his apartment. He always made sure to lock the doors and windows when he left, but still things went missing. One time, he was locked out of his apartment and the police came and found a hole in the ceiling that led to the attic of the three-story building. A homeless person had been living up in there. The thief had been sneaking in from the roof top into the attic and then down into the apartments, stealing what he needed. (LINK)
There are plenty of structures with crawlspaces, hidden rooms for hiding valuables, panic rooms, torture rooms, escape route hallways, and the like. Sometimes, when people stumble upon these voids, they have to wonder; what were they used for? If walls could talk!
Movies with the hidden room/hider themes
Bad Ronald: (1974) A perverted teenage boy who lives in the walls of a house finds the house sold to a family after his mother dies, and he falls for one of the new residents.
Crawlspace: (1986) A man who runs an apartment house for women is the demented son of a Nazi surgeon who has the house equipped with secret passageways, hidden rooms and torture and murder devices.
The People Under the Stairs: (1991) Two adults and a juvenile burglar break into a house occupied by a brother and sister and their stolen children and can't escape.
Hider in the House: (1989) A deranged man hides in the attic of a new house and becomes obsessed with the unsuspecting family that moves in.
Nightmare on the 13th Floor: (1990) A reporter investigates the disappearances of guests at a hotel which she is writing a report about. Along the way, she will find an unmarked 13th floor where a murderer is on the loose.
The Shuddered Room: (1967) A series of horrific murders is traced to a creature that inhabits a very strange house.
The Resident: (2011) When a young doctor suspects she may not be alone in her new Brooklyn loft, she learns that her landlord has formed a frightening obsession with her.
Through the Eyes of a Killer: (1992) A woman has a brief affair with the contractor who is renovating her apartment and he refuses to accept the end of the relationship.
Christina's House: (2000) The teenager Christina, her younger brother Bobby, and their father James Tarling moves to a rented house in a small town in Washington to be close to Christina's mother Joanne, who is interned in an asylum. The house is being repaired by Howie Rhodes and Christina has a boy friend, the bad boy Eddy Duncan. While in the house, Christina hears noises and has a permanent feeling that another person lives there, but her father believes it is the structure being settled. When two teenagers vanishes, the weird sheriff Mark Sklar visits the Tarling family, since the last place the teenagers were seen alive was nearby their home. When a dead body is found in a river close to the house, the sheriff investigates the place, trying to find the guilty for the crime.
The Changeling: (1980) A man staying at a secluded historical mansion finds himself being haunted by the presence of a spectre.
The Queen Mary workers recently found a secret room on the ship. They believe it was closed off for conversion to a hotel, but the machinery room is quite interesting!