OP here, I think that with Noct's ticks, his reactions to the world outside of the Citadel, even after he moved out, make it fairly noticeable that he is a sheltered rich/noble kid.

Granted there is a difference between a noble and the Crown Prince, but the Glaive wasn't completely unaware. Plus he wears black for the most part, and no one but the Line of Lucis and their sworn can wear that color. Though granted he could be wearing civies (not black) but I'm not sure how comfortable he would be in other colors.