I haven't had this problem on 307 but other pugs with similar complaints have been leaks from the filler neck to tank. Breather lines going into the top of the tank and a broken pump/sender securing ring. Access is under the back seat right side, though not absolutely certain on 307 as I've yet to do one.

Some pugs in oz leak from the plastic to rubber joiners from the fuel lines in right side of the engine bay, very common on 306.

However all of these did give clues with stains on the bottom and sides of the tank etc.

I agree that they can be difficult to find.

Hope this helps.

Mark