Gas smell when vents in fresh air mode

Whenever the vents are in fresh air mode, there is a smell of gasoline in the car. It doesn't happen when the vents are on recirculate. I first noticed this issue after the car underwent its 150,000 mile maintenance, which included replacing the spark plugs and the cabin air filter. I've repeatedly taken it back to the original mechanic (who is trustworthy), and they've confirmed that the spark plugs are tight and the cabin air filter is installed correctly. I most recently left it with them for a week, and they did find a gas leak near one of the injectors, and replaced it. They thought that would fix the problem, but it did not. I've also taken the car to a dealer, but they've been unable to figure out the cause either. There are no pools of gas under the car after it's left for a while, so it's not a large leak. Does anyone know of any especially likely sources of gas leak in this particular model, or, failing that, could recommend an especially tenacious mechanic, or one that specializes in these sorts of hard-to-find leaks, in the New York/New Jersey area?

Thanks in advance.