Who the hell is living inside my wall?

The Story: 

"I'm currently staying at a friends house.. Here's why.
I moved into a house about a year ago. The previous owners built the house in the early 2000's, and moved to India two years ago. This picture above shows my parents' room. Then the other day, we found this.

who is living inside my walls
What the fuck.
My little brother and me were messing around, and I pushed him into the shelf. When he fell, he pulled out the bookshelf with him. We always assumed it was just built into the wall.
There's a fucking spiral staircase in here. So we went down.. bad idea.

who is living inside my walls
Here's a shot of what it looked like from the top. At the bottom, the stairs led into a wall. Seemingly pointless, right?
This crawlspace was about halfway down. It looked empty, but if you crawled in..