StealthZ

First don't polish it out your self, even if your a great detailer. This is the dealer/Nissan's problem that they can resolve for you. I have Silver also and know what your talking about. I had to have the paint fixed on my hatch for that. I also had some on the hood as well.
From many discussions with the manager at a dealer I know very well, what he said is it is left over residue from the protective film (glue that adheres it) that they install during shipment, and that gets "baked" per say into the clear. He said it is very common on, you got it, the silver cars and mostly only seen between the Z's and GT-R's. (I have personally seen this on a show room GT-R as well)
Try and get the dealer to cover it, don't get the answer "well you can try and buff it out" Yeah OK , and what if you mess it up? Not covered... Dealer/Nissan should cover this issue. sometimes a light polish can do it or like my hatch, it needed re sprayed but put it on their dime, not your own.
Good luck on getting it fixed.