Pull the rear bench seat out and look to see if the fuel sending unit has pooled some gas. The unit is in the tank itself. There was a recall for earlier E55 AMGs but this has been cropping up on some non AMG E Class cars, too. afaik, there is no recall campaign on the non-AMG E Class.
The design was to comply with the EPA mandate of a returnless fuel line (where the pressure regulator and sender and filter are all one unit in the gas tank itself) so that fuel doesn't evaporate into the atmosphere. In some of the E Class, the top of the unit cracks and/or the seal fails.
I'd take it to a dealer and have them check it. If it is one of the sending units, you may be able to get a goodwill repair from MB since it's a known issue.
fwiw, the EPA mandate started in 2004. Some BMWs also had a simliar leak issue under the rear seat.
EDIT: ps, you should post this in the W211 boards instead of the W212 since you have a 2006 E350. You'll get better/more responses. This board is for the 2010+ E Class cars.
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