The rubber tube fuel fill overflow is a direct line from the filler well to the wheel well exit. In the roadster you can put the top half up. With the tonneau cover open, you can reach in and feel the rubber tube all the way down next to the fender. I had a problem where water wasn't draining from the filler after I washed the car. Tried running a chase and it wouldn't go thru so I had to get it all apart. Turns out the drain hose was kinked and tucked up in the wheel well and pinched. Straightened it out and it hasn't been an issue since. I brought the car home from Arizona so it may have been built that way, it looked like it. Little rain in Arizona and everything evaporates so fast. From what I could see it sounds like as dedwards 0323 said, may be fuel additive, even fuel overflow. Open your gas filler and pour some water in your gas filler area with the cap on of course. It should drain straight through. At least you will know if it's clear and narrow that area to the source of the ":stain" if it comes out in the same area or not. If it is the same area I wouldn't worry about it and wait for a re-occurrence. Keep an eye on it.