[Mr.G]

Ask her if it's self cleaning glass, or ever been coated with that water repellant spray.
The coating eventually wears away which could leave brush marks as it's removed, perhaps.

[pikeman]

That's a thought mr g that's what it looks like. Its a quite old patio though how longs self cleaning glass been about.

[rg1]

wet and dry  


Leave well alone.

There's no way you can scratch glass with a proper window cleaning brush, if it was grit you would have ruined every house that day.

Totally agree.