Should I be concerned about rusty nails just inside an outer wall?
You have to gather more information about how often the water gets there. Nails without coating (like zinc or anything else) will get rusty if you just wipe them with damp cloth and leave to dry. So nails being rusty only indicate that there was water at some point of time - it might be anything ranging from one time vapor condensation to washing floors to some major periodic leak.