Hi Magnus

 

How about going back and sanding an area to see how deep the brown goes?  It could easily be that it turned color without the sun exposure.  If left outside very long, wind blown dirt would accumulate in hollows and would leave stains where water sat.  Look for those.  I don't consider sunlight exposure to be a big deal on carbon parts since the carbon is completely opaque, it only affects the surface.  I have a new carbon rudder I built for my race boat that has been sitting outside for awhile and you can see the yellowing of the exterior but the solid carbon of the post should be unchanged.

 

Dan