It's the white blob, actually it's not alone.
And it's only visible from the rear of the model.
When the vehicle is animated, you know that engine running thing they do, that white blob moves back and forth but only within the confines of that red circle that i drew. The exact width in fact of the Templar icon.
All i know is that it's driving me nuts trying to get rid of it.
P.S. I deleted the alpha layer just as an experiment whilst trying to fix this and the top and left sides went transparent revealing what looked like a skined interior to the troop section.
I wonder if Relic at one point wanted the rear door to open and close.
P.P.S. That ugly blob on the door is what i think Relic calls a door handle.
I like your idea about the dirt layer, gonna have a look at that right now.
*Edit*
Well i've found whats causing it. It seems to be when any sort of colour gets to near that seam that runs down the centre of the roof. And it happens on the colour layers as well just not so badly.
This is as close as i can get without the dreaded colour blob rearing it's ugly head......weird. This means i have several choices, ditch the roof Icon, keep it and ignore the blobs or change the large icon into a smaller one somewhere else on the top away from that damn hatch.