Metallic "scratch" on inside of door window

[oliverjp]

I lent my car to my brother and now there's some sort of scratch on the inside of the drive side window. I'm speculating that somehow the seatbelt buckle is what did it. It's not really a scratch, I don't think there's any indentation, it's more like a pencil line was drawn on the window.

It's a BMW and I think there may be some sort of coating on the inside so while I'd like to get rid of the scratch, I don't want to ruin any potential coating.

What would be the best way to sort this?