Why did no one else in the arena notice that the princess was communicating with her lover?
Frank Stockton's "The Lady or the Tiger?" is a familiar and often-anthologized short story, one people talk about years after reading it. This semi-barbaric kingdom with its semi-barbaric king and his daughter, the Princess, is the home of the arena. Justice is meted out with perfect equanimity by the unflappable King. People come to the arena with anticipation, knowing they will see one of two things--a most joyous wedding or a vicious mauling by a tiger. These are semi-barbaric people who are in attendance to see a spectacle.
As for your question, then, can you imagine being in the audience at that moment: sitting on the edge of your seat, striving to catch a glimpse of this fearless young man who dared to love the Princess, wondering what he's going to do. Discovering the secret has never been done, so you have no reason to look at her; it's him who will soon seal his own fate, and you can't take your eyes off of him. It's no wonder no one notices the tiny gesture, as it's juxtaposed with the grand gesture to come--opening a door and meeting fate.