A hidden part of the apartment.

I was visiting my friend’s apartment and somehow she mentioned that she had such an interesting wall. The building itself is "small family project" series with 5 floors. Everything is made of reinforced concrete. But its interesting wall is distinctly like gypsy. That’s what I thought, and the apartment is in the very corner of the building, so there were questions about what was behind that wall.
It was also said that every day at 8 in the morning an alarm goes off from that place, which seems strange, because there really is no way to get inside.
The owner of the building knows nothing about this.

I looked up the plan of the apartment on the internet and really, there should be a continuation of the room. We thought of drilling a hole in the wall and using an endoscope to see what was there. Later I drilled a bigger hole for a better look. The owner of the building has allowed nails to be hammered into that wall, so we didn’t worry too much about small holes that can easily be covered with a taper.

There is a bed, a poster, a nightstand and all sorts of other things. All this only gave us more questions. Why are all kinds of things piled up there?
Why is the plaster wall simply built in?
Maybe someone was too lazy to renovate that part, but then where did they find the will to build a plaster wall?
Maybe for a smaller bill? But no one else could use that part of the apartment.

This whole situation seems so incomprehensible. That’s why I wanted to share and also ask if anyone here has the slightest idea why half of the room is closed with a plaster wall?