Stainless steel does not mean that it is impervious to rust. Stainless steel if exposed to moisture can develop surface oxidisation, but generally speaking this will not affect the integrity of the hook. Also there are different grades of stainless steel too, so perhaps the hook batch could be faulty or of an inferior quality. You mention high humidity as a contributing factor, and that sounds plausible. With all my reels be they spin, or fly reels, I have always maintained them by spraying them with a light coating of a product called Lanox or Inox, to prevent corrosion to the surface of my reels after every use in salt water. Perhaps applying a product like this and spraying on a rag then wiping your hooks onto the moistened rags will provide a protective coat over the hooks.