Mines does it too, it's the moisture coming off the coil when you turn the air con off. It's basically dampness, if it's on windscreen you will see the windscreen mist up. Doesn't bother me that much to do something about it, I'm an ac engineer and a good clean of the coil will sort it but I really can't be arsed. One of those things where you know how to do it but really can't be bothered