At first, the underground workers who stumbled upon this scene in a disused U-Bahn tunnel in Berlin’s Reinickendorf district in mid-January assumed they had encountered an abandoned film set. But checks showed that no film recordings had been applied for in the area.
Because no one would build a film set in a disused subway and then abandon it without filing the proper papers!
There's actually an organization that specializes in tours of Berlin's underground architecture and offers some interesting guided tours (although they probably wouldn't include the subway bedrooms): http://berliner-unterwelten.de/home.1.1.html
I've seen a short documentary about this - urban explorers who squat in unusual places in Europe (I think it was a Nordic country, but I can't remember the title).
Because no one would build a film set in a disused subway and then abandon it without filing the proper papers!