As you say, it is not common for the SV gauge to fog up. Has the bike ever been crashed? My guess is the housing might have been compromised somehow, letting a tiny bit of water in.... I looked it up online, and it didn't seem like a frequent problem. One guy on a duc said it happens on his monster, but it goes away after a few dry days.
If the problem persists, I would disassemble the housing (easy, just 5 screws hold it together), and inspect for damage/leak path. The SV gauge has vent holes on the back to allow moisture to escape, you don't want to plug those.