I doubt if its any rubber hose leaking or you'd have noticed the fuel puddle whenever the car stops.
I suspect with a high probability that it could be due to the rubber ventilators located at the rear boot needing some attention (read "replacement"). The exhaust fumes from the rich fuel mixture (most noticeable when at stationary) would somehow sneak through this rubber ventilators and thus leave its petrol smell in the trunk, and seeping through to the cabin.
Do check out this little theory ...
