It's the wall off the corner of the red entrance to the main room. The one with the decorative column in the corner. In a game I tried to toss a frag off of it and have it ricochet but it went right through the wall. I thought that maybe it was a glitch for that match only but later, in a different match I tested the wall again. I fired a smaw shell at it and it went right through. You'll notice that no bullet holes appear on it when you fire a scar at it. I was able to jump through it and it made me a suicide. I don't know if it has always been like that.
Maybe you suicided because it was filled with all those grenades you've been throwing in there, LOL.
Seriously, thanks for posting this. I had no idea. Now I'm gonna fill it up with grenades myself!