Thats the same exact spot where every Honda rusts. Its not one of their brightest ideas, but if you look up under your wheel well where the rust is coming through, you will find this like sponge type material. That holds the water and dirt that drains from the back window. Add to that those little black rubber strips and it traps everything.
Get it taken care of before it gets like mine.....both sides, roughly same spot yours is....$900 bucks to get it fixed correctly (cut & weld) and the paint blended in...
