Those are part of the AC system. There are no missing tiles.
Any marines hanging around the board? I don't know why but I just noticed the soldier on the left almost seems to be smiling and his cover is noticeably sitting higher showing his eyes. Even if the image is a composite, don't the Marines have a more exact spec when it comes to, well, everything?
I'm not implying that it means anything, it just struck me as odd.
They chirp like crazy AFTER a storm, as well.