Hello...

As i said.. with default settings and well done maps the server only sends information about other players/items if they could potentially be visible to the client. So the client can use transparent textures, wireframe or what ever he wants, the engine will simply does NOT KNOW that there is something behind the wall and as it does not have a build in crystal ball it simply can't show/draw anything

I'll try to show that via some images later when i find some time.. or maybe someone else wants to have fun..