Diagnosing a slow fuel leak

First thing I would ask is have you noticed the fuel level running down after sitting for a while? Also check the filler neck grommet, it could be old/shrunk/cracked/missing allowing gas to evaporate from the tank or fuel to slosh out while in motion (which you wouldn't see when examining the parked vehicle).

Do this - back down the driveway a few feet and hit the brakes hard so it's a hard stop. Put it in park and real quick, while the gas in the tank is still sloshing around, run back and see if you see wetness anywhere around the tank.