Actually the rust you are speaking of is probably caused by brake dust. Yeah.. Seriously. Semi metalic brake dust can build up and rust.
There have been a few isolated cases amongst the Xterra community of actuall rust spots on the hatch. These are permanent, and required repaining by the dealership. Closely inspect your hatch for these. If nothing is found., you should be safe.
Make sure you wax your truck, and wash it, especially the back hatch, often if you are experiencing the brake dust rust spots.
Also, just to let you know, this subject has been thoroughly disscussed in multiple forums. Please use the search function to find more information on this topic.
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:wq!
wqbang@southeastxterraclub.org