Why does Atub, Illusionist trainer from Largashbur, have a Northwatch Captain's key? Nothing at UESPwiki suggests a reason. Her entry there just notes she has one. Is she secretly a Thalmor agent? Anyone have any clues?

Maybe just an oversight. Same reason why a Falmer in Raldbthar has a key to Ikirthintag (what ever the hell its called)

Poopie.
I was hoping she was super secret agent that you found out about in some later quest.