Raw gas smell

Two times a month I drive for Meals On Wheels. While waiting for my door-runner to deliver meals I patiently wait with engine idling. During that idle at times, I smell raw gas. However, when I pull away I don't see a drop of anything wet on the ground. When I park back in my garage there isn't a hint of any gas smell. I have about 118,000 mi. on the clock and otherwise, the truck runs like a Toyota! Last tank I calculated 25+ mpg but that included a cpl of trips to visit a friend 20 mi away just off a 4 lane with no extra stops. No smell on highway driving. Ideas??