Nah, somebody got the white door at the far end of the corridor (not the phone room) to glitch and disappear, and then he could see through it. There's an entire modelled room back there, like a big barn/attic space with beams and stuff.
There's a video of it like... 100 pages back
No, there's a room on other side of the wall in the phone room. People have seen thanks to a glitch. It is fully textured, has full lighting, and is featured in the trailers.
It's completely normal for a demo to have areas like that. Take this opportunity to improve your knowledge of game development instead of continuing this embarrassing charade.