I had that problem too and it turned out to be whenever my wife filled the tank she didn't put the cap on tight enough so when I took a left corner the gas would splash out into the gas cap door area and smell up the inside of the car. Try and isolate the occurence of the smell and check for things like I've described here too.
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (194,000)
1999 E300 Turbodiesel (244,000 miles, dies at 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles)
2011 Audi A4 Avant (129,000 miles) hate this car!
BMW 330 xi 6 speed manual (100,034 miles)