The real cause is very likely to be both skin-oil from contact with fingers, and exposure to direct sunlight. Sun and oil together (along with high temperatures) can cause this type of thing quite easily. Also, other chemicals on one's fingers can do crazy things to plastic. I recall once having touched one of the covers for my N6230 with suntan lotion on my hands. The chemicals and sunlight together turned the clear plastic to a foggy milk-white, and the blue to a dark purple in a matter of a couple hours. That gave me pause about using said lotion the rest of that summer.