This sometimes happens on cold, damp evenings when there's a lot of moisture in the atmosphere. Slight condensation on the lens cover will indeed make images look misty.
CCTV cameras are available that have built-in heaters to combat this effect and here's the 'but';
A misty image isn't just down to condensation on the lens cover. You have to bear in mind that the moisture is in the atmosphere and as such, no imaging system can do much about the ambient environment.