Lately I hit my guard horn button a few time by accident, I've noticed that the notification showing that a guard horn have been heard in ____ area doesn't appear.
A few days ago I used the horn by accident, I asked someone who was in Vallaki, he was one transition away from me, but he didn't receive any notification.
I know that the NPC horn works just fine because they keep using it when they get attack by werewolf at night.