For Downton Abbey, I imagined Evelyn Napier was a spy. Based on his cagey answers about what he did in the Great War - something about being in Navy offices in London, yet someone else mentioned he was wounded in action - along the lines of Ian Fleming and his fictional creation James Bond, both of whom were nominally Royal Navy HQ officers but of course actually spies.
It made this occasionally recurring character, who was involved in some diplomatic/spy like things, including accompanying Mr. Pamuk in the pre-War period when the UK was courting Turkey - a little more interesting. IIRC, he was also involved with Bates in stealing a letter that was a problem for the Crown Prince, and alerted Mary to the rumor spreading about her and traced it back to her sister.
That it was never actually revealed that he’s a spy - and it never even crossed the minds of any other character - actually fits fine.