Why didnt your mechanic get the car on the lift and look for where the leak was coming from? The only answer would be that he did that, and couldn't find anything which is possible if its evaporating. But if that were the case, you shouldn't be getting drops on the floor of your garage either. Take it back, or to another shop, do what twins says isolate where the leak is dropping from on a piece of paper or card board. Then you can tell them thats where its dropping from. You also said your coolant level is not getting any lower? I'm assuming you are absolutely sure it is coolant and not another fluid..