In the current version? Wait until he leaves, then go past his position, go right and down the stairs. There's a safe room there.how escape at here? is it possible?
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After that, go across the large area into the large room on the lower floor. Crawl through the vent and there's another safe room with the bow. Once you get that, you can just kill a couple of the guards with the bow, steal their rifles and just shoot the rest.
The earlier version with the "Broken" stealth system I have no idea since there's a guard in the way and the way they patrol makes it seem impossible to even get to the first safe room.