To pbroad,
I think if you read through this thread you will find that most posts refer to one of two problems or both.
1. Areas of galvanic corrosion on the front section of the roof, above the windscreen due to a problem in the alloy composition. This whole panel is being replaced if under 6 years old.
2. Areas of galvanic corrosion on the aluminium elevating roof under or around the rubber seal. Affected vehicles have a respray but with, I gather, a thicker zinc based primer.
However, in your case it seems to be on the underside of the elevating roof section, at each corner. Different to the others.
I would get the vehicle checked, take photos and if the Dealership refuse to forward a claim under the 12 year Corrosion Warranty then contact VW Customer Services direct.