Bit of an odd one. My front screen is marked from where it's got condensation on it repeatedly while it's been off the road the last few weeks.. nothing will touch it.
I've not given it now then a wipe since it was fitted.. could it have had some thing wiped/sprayed over it during fitting? I've tried regular class cleaner and alcohol on a cloth so far.
Any ideas? It's definitely on the inside
Have you tried white vinegar, it may be a limescale deposit, failing that it maybe an acid mark ... and will need a cutting compound...