I didn't notice this until a few minutes ago. I have no idea how it happened, but it happened in the last few hours. I don't have a recent save to go back to. My most recent save will set me back about 40 hours of gameplay.
I know for a fact that I didn't sell it, so the only thing that I could guess is that I dropped it on accident somewhere. However, many of the places that I was playing in cannot be accessed again (The Kataliah, Castle Dour Tower for Dark Brotherhood quest)
There are so many things that I accomplished I really don't want to do again. But I also can't stand being without a complete set.
Is there really no way to replace the Nightingale set?
BTW, I'm playing on an xbox 360, so I can't just spawn another one.

Can I just kill/pickpocket Brynjolf or Karliah? I'm not opposed to this.