Good question. I have the same problem with my 4 month old 5559. The description in the reference guide says "Indicates the power and battery‑charge status. Press Fn+H to toggle this light between hard‑drive activity light and battery‑status light." If I press Fn+H, sure enough it shows hard drive activity.
Confession time: I never saw this in the documentation until I looked it up to reply to your question today! I never knew I had a disk activity light - man, I'm glad to have found this! 🙂
I have the same problem with my new Dell Inspiron 5558. So answer is: There is no LED in power button. Somebody in Dell forgot.. 😄