All four of my wheel hubs have rusty marks. This was apparent from a couple of months after taking delivery. The dealer said they were going to contact VW about it and would let me know, but I heard nothing. I think it's due to an interaction between the steel hub and the alloy wheel, but I'm not a metallurgist...
I'm planning to take it up with the dealer when the car goes in for its first service, as this is definitely a warranty issue