Other Fuel System Tank Mounting problems of the 2009 Subaru Forester

The check engine light came on while driving and stayed on. That meant something was amiss with the emissions control system, probably a loose gas cap. I checked the cap, took it off and retightened it until I heard the proper clicking sound. The light remained on the rest of the trip home. Twice more before servicing, I repeated the process of checking the cap. Both times I heard the clicking sound. The light remained on. I had the problem looked at by a Subaru dealer. After 2 hours they told me I was either overfilling the tank - I never top off - or not putting the cap on properly. I told them I know how to put a gas cap on, explained how I do it, and if there is a better way, please show me. They only repeated that it must be me because the cap was loose enough to be leaking gas. Despite my insisting that something else was wrong, they said bring it back if it happens again. I will repeat that the gas cap was put on as properly as it can be done. There is something else going on here and it is potentially dangerous. I replaced the old cap with a new one and have not had a problem.