Walter,
I have not seen this before, but I don't think I've seen any radar returns on the night vector screen before either. When I first saw the radar returns, they were very transparent, but then began to brighten up, sometimes all the returns and other times just part of the screen. I'm pretty sure when I panned and zoomed that it did not affect the brightness. I didn't think to check what the opacity was set at either.
Attached is another photo zoomed in closer than the others.
David