A small leak wouldn't really be noticeable to your MPG.
A little drip goes a long way as far as smell goes.
Outside air comes from the cowl at the bottom of the windshield.
The vents will circulate air throughout the cabin, so it's not impossible that it originates from behind you, but since you smell it outside the cabin, and the whole system is outside the cabin, that's a good place to start looking.
Look at the manuals where the fuel lines run. Follow them from fuel rails to the tank.
I personally have never had to deal with fuel lines yet, so I can't speak from experience, but they're just linear hoses and lines and I'm pretty sure I've inadvertently seen them just by being under the car a jillion times.
As far as what to look for, I would assume you will have wet, possibly even cleaned off in an area of prolonged fuel exposure.
Start off with the ground under the parked car. Any wet spots? If it hasn't been raining, spots should be apparent, but even if the ground is wet, look for rainbows on the wet ground. Your drip will be coming from above that spot somewhere.
Inspect under the car when it is dry out.
Look (while sniffing) for wet metal lines, rubber lines, especially where rubber turns to metal junctions.
And oh yeah...use a flashlight, not a lighter