Quite simply put; the tablet at the back is reflecting the doorway into nothingness on the opposite side of the room, where I've placed the camera. Seeing as how there is no 'sky' in the model, the default environment comes up black. Thus creating a dark shadowy area over the symbol.
I'm also lacking actual texture maps, so I'm having to use greeble a lot. I quite like the effect, and it fits in well with the other piece I did a long time ago of the same building. Though excessive use of it probably does make it look lego-y. Sorry.
