The answers so far regarding exhaust fumes while in traffic has merit.
Gas fumes on the other hand require further investigation...it will only get worse!
I have had two experiences with this...the SL had a tremendously bad gas smell at one time (especally with the top down), but it was easily detected as gas trickled out from the bottom of the car. Replaced a bad fuel hose clamp and everything was fine.
Just in the past month, the same occurred with the 300E. But could not detect a leak and nothing dripped when I started the car and inspected the fuel pump area. So I cleaned everything up and tested a few days later. This time, I saw a very small damp area on one of the fuel hoses. So I replaced it and now the fumes are gone.
It's amazing how little gas it takes to announce itself!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car