My S3 had same issues in much the same places plus more within the first year. The car was returned to the factory under warranty and virtually resprayed. Needless to say that the same problems recurred within 18 months and I believe now as Aero owners it is something we have to live with. The issue is under the paint and is more likely to be some sort of reaction in the aluminium that is not associated with salt.
I plan to have panels resprayed as and when the stone chips get too bad and hopefully the corrosion can be repaired again then.
I would be interested to hear of any other solutions
