Wow. I have panel for 3 years now, never cleaned them actively (even if the installers offered me the service for a nice monthly fee), and never noticed anything like that. Well, let's wait 1-2 months to see if it's still the case, at least (April/May is the best month as far as production is concerned)
They're super easy to clean if you can get to them. Just 30 seconds with a regular household mop and they're shining again.
I'd recommend doing it at night, because I got a nasty zap off mine when presumably some of the mopping water got into the electrics and came up the metal mop pole .. (the electrics are all supposedly fully waterproof, but I guess on my 10+ year old panels stuff has degraded)
