Theory - moisture carrying the salt hits the outside of the window, drips down, pools in the channel/frame that the window sits in, and comes up the inside. Splashes up the inside.
As to the small area, is the window design new?
I offered a possible solution to the SM. Take the window out of the molding and put a bead of silacone along the bottom of the window and put it all back together. He refused to do it because he wouldn't get paid.