Yep fog when I notice it at all is usually when eliquid is dipping low. It kind of makes sense. Around the time it's so low, I start to notice the vape quality changing and need to refill. But, I am MTL I could see that happening earlier with more heat and etc. Also, condensation CAN'T form if the tank is pretty full, there would be little room for it to do so, be visible and etc.
Anna