It depends. The example as provided is incomplete, so it's hard to say. This is why I've offered a few ways to handle it, depending on the circumstances.
For me, if player-driven play is a priority for me, I would have the trait of the guard be known. I'd either show the players in some way, or I'd let them know it. This is assuming the guard is not meant to be a meaningful obstacle in and of himself, but rather a step toward a more meaningful goal. I'm not going to spend a lot of time on such an interaction and just getting it to the point where it's actionable by the players.