My first apartment in NYC was in a building of tenement style railroad flats with identical buildings on each side.
There was a fire escape outside the kitchen window. We shared this escape with the identical but mirrored apartment in the building next door. (I mean the apartment was a mirror image of ours, not that it was decorated with mirrors).
One winter our refrigerator broke. While we were waiting for resolution (which wasn’t a quick process because I was living in a tenement), we began storing our food on the fire escape.
One night I was in the kitchen and I heard a knock on the kitchen window. A woman I had never met was standing on the fire escape. I opened the window and invited her in. She introduced herself as the resident of the adjacent apartment in the building next door and she was concerned about our food storage because she liked to let her cat out on the fire escape.
So she offered to share her refrigerator with us. So, for the next couple of months I went out through the window and into her apartment whenever I needed the fridge. We actually became friends, which was a good thing since I was crawling through her kitchen window several times a day.
When I first read the story about the “secret apartment”, I thought about this set-up and just figured this lady had found a way into the building next door.