Playing last night on a populated server. Myself and friend decide to clear out the prison building at Vybor military compound. We find at least 5-6 dead bodies on the floor (which we struggle to move past). Finding the prison completely empty we start to move past the bottom of the stairs when a player moves out, crouched through the wall and kills my friend, I manage to shoot the player twice with an SKS and knock him unconscious however he sinks through the floor. I run upstairs and they are unable to kill me through the wall from downstairs to upstairs.
While upstairs I could hear that there were at least three players in the wall, the player I knocked unconscious slid under the floor and back outside however.

It looked as if the player was below the floor of the building while he moved out from the wall.

I know this was an issue quite a while ago with players in the walls of this building however it looks like they have found a way to get back into it downstairs.