The internuts has said, that its the filtering of light via the windows that causes the yellowing, if you have direct sunlight with windows with UV coatings, you can have problems.
The other issue I suppose is temperature, I know for a fact, I have had keyboards in storage in a sealed box, with no light I have taken out years later and they turned yellow. These were in my basement, so other than heat in the summer and freezing in the winter, not sure what caused that.
I have also experiment with direct sunlight on Famicoms and 1050 drives, amiga keyboards, etc they all lost a vast amount of the yellow, but not 100%. There is risk of course of making the plastic brittle from heat, but that is why even now in the cooler weather, this works with nice bright sun like we had in New England today.