Not actually my house, it was an AirBnB outside Seattle, but it's mysterious and I'm hoping someone here might know the function.
Inside the main closet in the master bedroom is a weird giant staircase leading to a square hole that looks into the area outside the bedroom. Here's the other side:
The inside of that cupboard beneath is just a small empty space, much smaller than the staircase, tiled at the bottom and possibly sized for an electric fireplace or something.
My friends and I could not figure out what it was supposed to be, and it's driving us a bit crazy.
That would likely be where I'd keep security camera recording boxes and my home network router/switches/media server if I lived there and ran ethernet outlets to all the rooms.