Bike stood in one position for some time? The pitting matches your pad profile by the look of it.
Classic grain boundary pitting, not a porous casting. These discs are made from profile cut stainless plate, Its caused by the material not being in the correct solution annealed condition, this causes migration of the nickel and chromium from the grain boundaries during the rolling process, leaving the boundaries of the grains lacking any of these protective alloys. Solution annealing should be the final part of the process. material is heated to1050 to 1150 Centigrade dependent on grade and quenched, putting the alloying materials back into solution, it is then pickled to remove any surface scale.
It basically rusts at the grain boundaries if left for any time opening up tiny pits as it goes. Stainless steel will rust, most noticeable on welded joints that have not been pickled and passivated to remove the surface iron after the heating process.
Not environmental, although you could dry your discs and turn the wheels regulary to remove moisture. Ask the dealer if he believes that 8,000 miles is the life of a disc. I personally think that properly solution annealed they should last a lot longer. Poor manufacturing process and quality control on raw material.
