Low Coolant Code


Last year I switched from DexCool to Green anti-freeze, everything has been fine until last month I got the "Low Coolant" code. I had to add about 1/2 gallon and the code stopped. Last week I was at a friend's house and after I left he called my cel and said there was anti-freeze in his driveway where I had parked.
I took the car to the mechanic and he couldn't find any leaks, I have a suspicion that he didn't put it on a lift though.. Yesterday I got the Low coolant code and it took another 1/2 Gal to fill it. There's no spillage in my garage or anywhere else I've parked it except at my friend's house that one time?
Someone said it could be my head gasket? Any ideas anyone......


