I think it's the exposure to light that is the main cause. It doens't have to be direct sunlight but just regular daylight can make them turn yellow in time.
I don't think smoke has anything to do because if that was the case a good cleaning product would able you to wipe it off.
The best you can do I think is to keep your carded figs in a dark place, that's the best way to prevent yellowing of the bubbles.
And yes those uv cases will help but if you can trust them 100% is still a question.