If its a sedan the likely first place to start is the short rubber hose that connects the filler pipe to the tank itself. They are randomly starting to perish and start to leak fumes and eventually to leak fuel itself.
I've replaced both hoses on my and Dad's BF. Dad's was bad, it looked like someone had taken a stanley knife to it, cutting slits along the length and was leaking fuel. Mine you could smell a faint fuel smell under the car on a hot day and only when you really squeezed the hose hard once removed could you see any sign of damage.
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If it is that, good luck finding one. They are very rare at the moment, rare as rocking horse ****. Doesn't help that the assholes at Ford decided to make the filler pipe and tank inlet different sizes, so you cant even used some standard hose to do the job.
Worst case you might have a crack in the tank or a seam where a fitting protrudes from the tank. Apparently this was pretty common at one point, tanks failing under warranty.
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The join around the 2 white circles on the left of the tank fail and fuel and vapour and fuel can leak from there. You don't want it to be this because you need to take the cradle out to get the tank out.
The carbon canister is under the raised section of the boot floor, right behind the seats. It's just off the drivers side of centre. You need to jack the car up and get in there with a light as its wedged up deep between the floor, cradle and fuel tank.
Start with the fuel filler hose, cross your fingers and hope its that.