only thing i can think of what i said before about the rain or carwash, water wont come out of the seam. actually if one of those coolant hoses under there is leaking it could be running down the hose and dripping off onto the same oil trail, making it milky. and if your like a million other people, you just add water to your radiator when its low and it got watered down enough to not leave the colored stain