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I've come across this a few times over the years. As others have said sounds like burns from a grinder. What I do now is bring it to their attention before they bring it to mine and tell them that unfortunately it can't be removed as the glass is damaged. They usually do understand
Has someone used a grinder near the window.
A similar effect occurs near railway lines. Tiny specks of steel from the rails and train wheels get thrown into the air, they fall on plastic sills and rust into the surface leaving a brown speckling all over. We do a row of houses that back onto the railway and they're all the same. No point in replacing as it will only start again.
I've had this before - funny thing is the plastic frames don't seem to get it