[Stu baru]

Originally Posted by bestwheelbase

The glass is tempered (so it shatters into tiny bits rather than throat-slicing shards ) and what you see is a byproduct of the manufacturing process. Specifically the cooling phase, which puts the glass in tension. Polarized sunglasses make it more apparent (depending how you turn the lens), but often it is visible with the naked eye.

Bingo. I'd never seen this on the side windows of a car before my BRZ, but often on rear glass of many makes. WAY annoying in the side glass since I only use polarized sunglasses (and you should too). The term for the optical phenomenon is birefringence.