Why are my pieces so yellowed in zero sunlight?

These sets have been displayed in closed off rooms for around 5 years, and almost every white piece has suffered severe yellowing. The downtown diner especially having some of the pieces starting to look like the tan color, and on the side of the building that I’ve had displayed connected to other modulars has yellowed bricks as well with no possibility of light even getting to that side. Just curious what’s going on here cause I know sunlight isn’t the issue. submitted by /u/Sirbanza ...