Our banding was only riveted on with pop rivets, so if they're not sealed at the back, they could potentially leak. We also had a piece of banding fall off while she was still sitting in the yard being worked on. That prompted us to decide to completely re-rivet all the banding before taking her on the road. We used closed end pop rivets to do this, drilling out a few and replacing before going on to the next set. Took me most of an afternoon, but it's now all solid and sealed.
Water might also potentially travel from other areas, such as the curved area above your window. Might be a long shot, but people have had water show up on the floor that came from the roof and down a wall. Water will always seek the path of least resistence and if there's a small leak up there, it could run down the outside wall till it hits insulation to soak into. Stranger things have happened!