I wanted to test my own theory that it only happens when the tank is full. So I'm going to do a few half fills over the next few weeks and see if the smell goes away. Then it's probably off to the mechanics for a deeper look.
I get this only when I fill up to full and some of the gas spills outside of the filling area. It normally drains out of the small hole below the cap(in the gas cap area) through a tube to the outside underneath the car. Unfortunately some of gas lines up this tube and eventually evaporates.
When I wash the car some of the water falls into the hole and washes the gas remnants away.