Yes looks like a skip spot when they welded it. But now you have a good look at things looking nice in there. You can see how the did there construction. Put the sealer over the bare seam. Did you find any primer in there. I would not be surprised if all that was in there was over spray from doing the body shell. All time and money to them on something that was going to get covered up. You can tack that hole shut. Its these places I like to use brass on to closed. A lot of advantages there. cherry re, rust is dead. then you fill it with a product that will not rust. But you can do as they did but add a layer of primer befor you fill it in with sealer.