Security camera and door bell clouding up at night

Had the doorbells and external camera for varying lengths but had no issues with clarity during the day or night. However after a weekend away and checking the cameras remotely (as you do) I noticed that I was getting an odd blur so I couldn’t really see what was going on, just a smudge of a picture. The following morning it seemed ok. It did the same the following night, I suspected that maybe a spider had made its web over the lens and was catching reflections at night (oddly on two different things in different places). When we got home I used a big duster on a stick to clean the camera and the door bells etc. Seemed ok that night,but it wasn’t as cold that night as it had been. A few nights later it did it again in the middle of the night (looking back at triggers) , cleaned again but it still did it the following night.Is it the lens losing focus and focusing on the glass ? Are they letting water in and its condensation? It never actually looks like its got a web over it, I should point out.Thanks