I used cheats to get enough ego to unlock everything. There is no ending behind the locked door. The only thing you find there is a telephone. If you interact with it, you get a text message saying "the map is not in this area" or something like that. I searched all the levels for whatever this map is supposed to be, never found anything.
Either I missed something and there is an ending nobody has ever seen, or this is just unfinished. Dev is anonymous so we can't ask them.