My focus does this my ranger did this. If it has colder air coming out of the vent even if the vent isn't on, than the air outside air it will build up at bottom of window I run the defrost on heat to match the outside air it will go away. Sometimes you can just be driving along on a back conrty road with a clear window then you hit a warm patch of air and the window will completely fog over in seconds .
Do one thing every day that scares you. -Eleanor Roosevelt