I found similar rust spots in my drip channel too, but not as bad. Its the weld seam between the roof and the side panels. The Expedition use a self leveling filler compound that gets painted, but a lot of vehicles use a black trim piece to cover the gap. I'll be taking my Expy in to have the apply touch up paint to the problem area. Its out of the cosmetic view, but I don't want it to spread.