Mist coming from A/C vent in my Hyundai Tucson?

While driving in January I took the car in because I saw a mist coming from my A/C vents while driving with it on. Took it in & off course they said nothing was wrong with it. Now I am smelling fumes like gas (I'm not sure what it is) from time to time from the A/C vents also and this morning I saw the mist again & turned off the A/C. I took my car to a mechanic & he said there is a leak causing the A/C to act that way. When I took it the dealership again they said nothing's wrong with it. Has anyone ever heard of that issue before or experienced it?