Pollen, mold spores, dust and pollution sticking to the morning dew on the lens, insects smashing into lens leaving pollen, moths leaving dust (actually micro-scales), smoke and vaporized grease from my grill and smokers… even rain doesn’t remove all this gunk. It’s not an issue until you live stream with sun directly in the lens or IR reflecting back into the lens at night. It’s kind of like getting old. When you see yourself and others all day every day, you don’t notice a small daily change. Until you do. 
So very glad to help you out! 