It was a sliding wooden door behind a woodworking shelf (the door blended perfectly with the handtool holder wall). The room was about 3m by 2m.
When we found it, it was empty and had hooks on the ceiling with two power outlets.
We didn't tell his parents and thought it was a "cool" after school hangout spot.
A few months later, we went back and a new shelf with weed plant cuttings was in the room. My buddy asked his dad and it turns out he had been growing a few weed plants hydroponically for the prior few years.
Someone I knew growing up really had a secret room in their house. Inside of a closet there was a wall that opened to a room that was really just the space between the walls of the room to access some ducting and the water heater. The secret door was definitely not obvious.
My brother and I went on a streak of trying to find hidden treasure. One weekend we convinced my grandfather to drive us around to scope out spots to look. My grandpa told us to give him directions where to go. Eventually we found a spot and started digging. After digging a few shallow holes my grandpa pointed to another spot and to our surprise there was something solid about a foot down. It was an old Neiman Marcus tin full of loose change and candy. We were thrilled and for years we would tell anyone who would listen that we had found buried treasure.
When I was an adult if finally occurred to me that my grandpa had set the entire thing up. There is just no way my grandparents would have let us eat candy we found buried in the ground. But he had set it up well enough that my brother and I didn't notice. We believed we had given him directions to the place it was buried and we believed we found it after searching the area thoroughly. Really, he had buried it earlier before we came over and gave us subtle hints until we found it. We were four and six at the time.
When I realized it had been set up I called my grandpa and we had a good laugh. He told me not to tell my brother before he hung up. Keeping the secret is key.
A friend of mine showed me a room he found under their house in a basement(which isn't common in South Africa) in the storeoom he found a bunch of unlicensed firearms. Think they belonged to his dad, but there were so full of dust even if the police had found them they might have thought they belonged to a previous owner.