i had the same after a few months of using the v700. There were also a few large white specks on the inner side of the glass, of unknown origins.
I *did* see the fog back on the scans, in dark regions of slides especially.
So i removed the four screws, took the glass off and cleaned it with a soft microfiber cloth. Dipped part of the cloth in isopropanol and wiped it then wiped it with the dry side too. Repeated a few times until it was sparkling clean.
After about a year or more, it started to fog again, so i repeated the procedure.
It's not a big deal. Just do it with a clean cloth and don't push too hard. And cover the scanner with something so it doesn't gather dust while open.
I will do it as many times as necessary. I didn't buy a 500 euro scanner to be lazy and hope that fog and junk on the glass won't be visible.