I'd really have to give that some thought. I can see pros and cons to it.
Even in policing, we have many officers who are not carrying guns on the outside of their clothing. Their guns are hidden. Undercover officers carry guns, and if faced with a dangerous situation in which they have to show the gun and take action with the gun—whether they actually have to fire it or not—they very quickly identify themselves as police. They have set things they yell out, like, “Police, don't move”, etc. They're specific commands so that people will know they're dealing with police officers.
I'm assuming it would have to be similar. I don't think it would take very long for the world to know, when pulled up to the customs booth, that the person in there was carrying a gun. In some circumstances, visitors from foreign countries might not expect that, but I think that would be more surprising. But if the visitor took aggressive action, they'd know fairly quickly. I think that's what would dictate the ultimate end result: how an officer handled that at the time and whether they made the appropriate notifications and went through the proper progressive steps of the use of force before they actually fired the gun.