I don't know if this means that the Nikon is afflicted with the same issues that affected a percentage of early Pentax K5s (mine included), given that the share a similar Sony sensor.
There has been a lot of chat about it on the Pentax forums - such as
HERE
Pentax have replaced all of the defective bodies, although they could have been more informative on their website.
The stains were only apparent at small apertures, f11 or above, and when photographing a plain background. So set your lens to f16, manually set the focus at infinity and shoot a picture of a clear sky or plain piece of paper.
I hope you get your problem sorted soon.