Gas smell inside of car.

I have 2005 dodge neon sxt. Its has a gas smell inside when the car is at an idle and I am stopped. It goes away once I start to move. I checked it out and could not find any leaks. I had my mechanic look at it. He could not find anything either. He checked the line from the tank to the engine, and the rail and injectors. The car runs fine. Any ideas?