I have the same car and had the same problem. Replace the cap. If that doesnt work check the fuel expansion tank and charcoal canister and connecting hoses. My problem was the spout on the lower side of the exp tank.(near the gas fill nozzle) had a crack in it. Good luck.
Also, any issues filling up at the pump? Do you top it off? I noticed gas leaking out of my car only when topping off. The gas was leaking out of said crack.