A recent personal experience could possibly shed some light on your potential problem. In retrospect, it was all so obvious.

Pay attention over time whether it takes just a little longer for the engine to turn over/catch each time you turn the key to start the engine. If the fuel line exiting the gas filter has aged enough to develop a pinhole leak, you will smell gas when the engine is turned off as the gas pressure in the line bleeds to atmospheric through the pinhole. Your experience of not seeing a gas puddle on the ground now may change 2 months from now. If the engine was not running (fuel line under pressure) when your mechanic examined the fuel line, chances are he would have missed the pinhole leak. The human nose is fairly sensitive to certain initial smells at low concentrations, gasoline being one of them.