Water droplets have to be present for ice to build up. Because of the cooling in the carb, unsaturated ambient air can become saturated and cause icing.
I know where you are comming from, we are taught that if you can't see the water, you won't get ice (except for hoar frost).
However, if the air humdity is high enough, then it is possible for the pressure drop in the venturi to be sufficient to condense the moisture in the air and so form droplets?