I have a Versant 280 and have the same background toner on the print side. It’s only noticeable if you compare it to another sheet of unprinted stock that has a high white level of brightness. On coated stock it’s more noticeable, on uncoated stock it’s almost impossible to see unless you’re looking for it and even then it’s hard to find. The image that you posted does seem to be more evident but it’s difficult to tell for sure.
The toner that creates this background is smaller than the yellow security dots and can only be seen through a loupe. I don’t know for sure but the tech believes it’s caused from floating toner dust within the machine that gets fused to the sheet. If that’s the case, then it would be best to make sure all your filters are clean and to vacuum any toner deposits that have accumulated within the press. I think the floating toner explanation is a stretch but possible. It's not a Fiery issue, you'll get the same results from a copy off the glass.
I have some test files that were printed by Xerox on a Versant 180 and they too have the background. I never noticed this background when I was getting samples from vendors but sure enough when I pulled out the samples over a year later I could see it if I used a loupe. If this background image is the same as I’m describing then there is no fix, it’s a built in Xerox feature.
