The big issue was some pest control anti-wasp spray that got underneath the eaves in the shed where I had the bike parked. Splashed on the windscreen (without me knowing it) and I've got some opaque areas that I'm not happy with.
Interesting. I also have a similar problem on one of my shields. There are some opaque splotches. I have no idea what got on it, but it woulldn't wash off. In an even more bizarre twist, in the past when it happened I thought they were permanent damage but they came off, accidentally, while cleaning the windscreen. But I can't remember what I used. I actually think it was Mother's paint polish... (the non-wax waxing stuff).