I have run into two other sources of yellowing: storage boxes and fluorescent lighting.
I had my Commodore and some Amiga stuff stored in boxes I took from the Taco Bell where I worked. Anywhere the box touched the plastic turned VERY yellow. I'm thinking it was an acid content issue.
Then in a room with NO direct sunlight but using a mini-fluorescent twist lamp, I watched some of my equipment yellow rather rapidly over the course of six to eight months. Normally one would attribute the discoloration more to age, however the patterns exactly matched how light fell on the units. So, I'm back to incandescent lamps.