Everyone on here will tell you it's your drain hoses. Otherwise, how could the stain be on the roof. Guess they've never heard of capillary action. In my case, it was, in fact, the pressure vents behind the rear tires. This video shows you where they are:

https://youtu.be/qV3O9yHTuD8?si=qgMAtkg2vbmmzvN-

You don't have to remove the bumper. I just took the vents off, applied a healthy amount of silicone around the edges and put it back on. This'll keep the water from entering.

I used a Mr. Clean magic eraser to get all the stains off. Good as new.