If the gas cap didn't fix it, then it's possible there is a leak in one of the plastic evap tubes, or the canister purge solenoid is stuck open, thats on the canister. These systems are tested with a smoke machine that pumps smoke into the evap system, and where the smoke comes out, there is a hole. The solenoid is fairly cheap. It's located on the underside of the car near gas tank. The purge valve is in this picture, and is where the smoke gets pumped in.