GAS CAP!!
This happened, I hate to admit it, but you WILL smell gas.
I put the gas cap on the gas pump (RT/10) and was talking to people which always happens when fueling the Viper. Folded the rubber flap and closed the gas door and drove off. My wife said " I small gas", I said " I smell gas". Pulled over and checked, only to find NO GAS CAP; WE SMELLED GAS!! Headed back to the gas station and guess what, it was still there on the pump. Now, you may not be this forgetful, but the cap could be loose on not sealing properly.
BTW I now hand the cap to my wife when filling up.
Good luck.