I still think it has something to do with the GI...
As we have seen in the past, the GI system has a mind of it's own, allways rendering scenes different. Oshyian warned about GI in animations, giving different lighting in every frame, or even with cropped and stitched images. Or what about the red renders?? Those are still showing up too, abort and render again, nothing is wrong.
Just this afternoon I had a render of a vulcanic scene. I had a dark gray base layer, a bright red voronoi pattern surface layer, and a white ash layer. On one of the 10 cropped renders I made (800x600 had to be rendered in 2 halves....) the red layer suddenly had all these orange spots. Rendered again nothing wrong.
All this points towards GI randomness, in this scene too.