Hi rob I have taken on a few new jobs recently and this sounds like the same problems as my customers are having when I examined the window, (have a look next time) I noticed that where some of the black chips were knocked off there were tiny little holes (chips) in the glass when I question to see if they new anything about it one said always been like it glass was fitted faulty, another believe it or not had a BBQ next to the window and another her dad done some grinding near the window and chipped the hell out of it but if the same as mine told them the glass was well damaged. But have a look the area you scraped just see if there are little chips in glass.
Alan