I had a similar problem on my w124. I would fill up, drive home a few miles (ie, there should now be a bit less fuel in the tank, right?), and maybe 2 days later there would a fuel leak under the car that was like 16 inches x 16 inches, ie, pretty big. Couldn't figure out what was going on.

Had my mechanic look at it, and there's a bell shaped thing under the fuel tank. you might be able to see it without having to jack the car up if you get your head in there and look around. I forget its function, handles some fuel overflow or something.

End of story, unless I wanted to do some work under the car (I didn't), the solution was to not fill up to the very brim. Somehow what was happening is that if I fill up completely, some of the excess fuel goes into that overflow hose and down to the bell shaped thing. Now that the car is 28 years old, something is not working like it's supposed to, pressure builds up, and that fuel overage ends up on the ground.

Solution for me: As soon as I hear the fuel pump getting to where it'll shut off soon, I stop pumping right then. Problem solved in my situation.