Well, With no trace of the smell at idle I would have to guess maybe a leak
on top of the tank or filler hose.
The "draft" or negative pressure behind and under the car is sucking the
fumes into the passenger compartment maybe?
Long shot but I've seen them leak from the rubber boot that connects the
filler pipe to the tank
and also leak from the ring that mounts the sending unit/pump assembly into
the top of the tank.
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