I was pondering a point in a moderately recent Girl Genius comic where Gil is addressed as "Your Highness" -- revealing things about his mother's side he had not known.
But what I was thinking was that he had to be merely a prince for that.
Except that I've also read Agatha H. and the Clockwork Princess. In which at one point Klaus calls Bang "Your Highness" and there was some amusing chit-chat about how a pirate queen (or king) would sometimes be treated as royalty by the chief families of Europe.
To be sure, the titles are not a stable thing. In medieval times, a king was "Your Grace"; only an emperor qualified as "Your Majesty." And title inflation, with grander nouns and throwing in a few splendiferous adjectives, is remarkably easy. But I don't think there was ever a time when Your Highness was the sovreign's title.
I may be wrong. Or that Europe may have see the inflation work a bit different. But this is one of the things that makes sf/fantasy mysteries so difficult to write. You don't know whether something is a clue about the world, or an anomaly in the world that's a clue about the crime. And it's a problem for other forms of foreshadowing and set-up too.