It is a letter. Just a letter which i ve been found for almost 10 hours. )
It lies in one special room.
Here is my vid:
(check from gameplay time 1h:04min - you can see timer in top-right corner)
But why i say "super secret"? It is just a letter! Becourse you can only find it if you know what you are looking for. There is no secret room splash screen in the place, no artifacts. But there is one small piece of storyline in this letter.
It is sad that somewhere there is a room with load of boxes. In this room only one barrel stands. There is a red rock in this barrel.