Mystery leak

I'm concerned about a fuel smell in my son's EL Ford station wagon. There is no smell as long as the windows are closed, but as soon as the passenger window is opened, there is a strong petrol smell. The car has been checked for leaks three times without finding any. My son has replaced the purge canister and hose as recommended by the Ford dealer without success. We are continuing in our quest to find the problem, and I'd like to ask you what it might be.