I realized this when my SNES yellowed evenly on only one type of the plastic used on it - the body. The cartridge slot plastic was left untouched, and looks new - while the entire body is yellowed perfectly evenly. No one in my family smokes, it was just left in a plastic bin with the top on it for years.
The same happened to my launch DC in another plastic bin. Although it was only in the bin for around 6 months, so it's only slightly yellowed.