Any Idea?

On the corner of every pane of glass on every bus is a British standard mark showing the specifications of said glass, and on every window on every bus I have been on in the past year someone has highlighted these with a yellow right angle. does anyone know why? I really want to know (well when I say really, obviously not enough to contact some one who might actually know and be able to tell me, but enough to put it up on internet so that the cyber flotsam and jetsam can speculate)