Your gas tank is under the rear seat floor. Your mechanic needs to look
along the WHOLE fuel system, including looking under the access plate to the
tank, under the rear seat.

Most of the fuel line is steel. The exceptions are at the fuel filter (first
area I'd examine), the return line coming off the fuel rail at the regulator
and the area Stewart mentions under the rear seat where rubber lines go from
the feed and return lines to fittings on the tank.
I'd first give the hoses in the filter area a really good inspection.
They're often buggered by folks who improperly remove and install fuel
filters.
With the key on but with the engine NOT running, the fuel system is
presurized by the pump. You should be able to locate any leaks.
Another area where a leak is possible is at the fuel injector, where it
enters the fuel rail. The o-rings used here sometimes fail.
I do not think it would be a problem with the vent and charcoal canister. I
don't even have a charcoal canister on my Eclipse. Smelling fuel as you do
leads me to belive that it is pressure/return line; the vent system wouldn't
smell so much.

This fuel pump instal shows the area under your seat:
http://www.vfaq.com/mods/pump-WB-2GFWD.html