My 2016 Bennington 21SX has developed a very strange, large stain on the stern deck forward of the motor and behind the rail. This portion of the boat is open to the weather. There is no rust dripping from above or anywhere on the boat. We’ve used cleaners, straight bleach, CLR plenty of elbow grease scrubbing it and even pressure washed it to no avail. It’s almost like it’s rusting from underneath? Bennington dealer/service is stumped. See photos below. Thoughts?