Are you sure none of the water wound up in your trunk when you did the test? One time after I filled up I smelled diesel in my trunk and found the hose that cconnects to the backside of the drain hole - which happens to be in your trunk - was disconnected. So any overfill wound up in my trunk. The hose was very hard and I could not get it back on the metal drain pipe so I heated it up and then it went on fine. When it cooled it shrunk tight and hard again.
Your situation is puzzling though, in that you had some water come out of the underside of your car - unless it came out of your trunk drain holes
Liquids have a tendency to travel under the plastic trunk covering so it is hard to see.
Just look in there with a flashlight and make sure the plastic tube is connected where it is supposed to be on the backside of your fuel filling area.