I was able to find a video [1] of the building, with that area visible to some extent. You can see it at various points, like at 3:43. I'm not sure of its significance though.

[1] http://www.youtube.com/watch?v=NX59-ZBO3mo

Looks like a door/access of some kind.

Edit: There was some kind of entrance tunnel connecting there : http://exposingthetruth.info/wp-content/uploads/2012/05/fuku...

Still not clear why they'd shop that.

Edit 2: this article has more details : http://translate.google.com/translate?sl=fr&tl=en&js...

It's the entrance of the tunnel from which fuel is normally loaded.

This looks like the work of Microsoft Paint to me.