2007 Chevrolet Aveo Antifreeze leak/car overheats

I found out in the summer of 2012 when the temps were above 90 in Texas that my car's water tank was empty and I had to refill it every week or two. It was not overheating and there were no engine lights that came on indicating a problem. In September 2012 I took it to the dealership ( I went to 2 different dealerships) before the warranty expired and they kept it for several weeks and did every test and also drove the car and it did not leak. At that time the temps had lowered to 60 degrees or less. It did not leak all winter until May when the temps outside rose to 80+ and I had to refill again every week or 2. At that time I was only driving my car 3 days a week and not long distances.

Currently the car's thermostat rises when idling at a red light for more then 20 seconds when it is 80+ outside. The hotter it is outside then the quicker the thermostat rises. When driving down the road the thermostat is normal. In June 2014 the oil light came on and I went to get the oil changed and the guy who changed it told me there is an oil leak and he changed my oil from 5W30 to 15W40. I don't know if the oil leak ahs anything to do with the water leak.

Friday I noticed after driving 30 miles that when I turned my car off and had came out of a restaurant after 30 minutes that water was dripping under my car. The next day I checked my water tank and it was empty so I refilled it and 4 days later I drove the same 30 miles and noticed when I shut the car off that I smelled Antifreeze and there was a puddle of it on the ground. Checked the water tank the next day and it had a little bit of antifreeze at the bottom ( was not dry).

So I don't know what to do. Should I go back to the dealership ? It is very frustrating that 2 different dealers looked at my car in 2012 and neither could find a problem.