The note can be found inside the Burrows, next to a corpse, at the old tunnel gate entrance. It cannot be taken.
Okay Pete, I didn't wanna write a note like I'm your damn mom or something but I'm getting sick of sending out crews to look for the keys you lost. You take something off the shelf, you put it back. Understand?
Put the keys back in the throne room before we have to beat some common sense into you.