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Could be an abrasion, or as you say a water mark. Perhaps a mould?
I have managed to restore those humps before with warm soapy water and a scourer pad.
Try the soapy water with a soft cloth first and if that doesn't shift it, very gently use the scourer in small circular motions over the whole surface as it will change the surface texture, so you have to do it evenly. It's good for dulling the shiny stripe where the cowl sits. but just go gently.
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