Several decades ago I was hired by a multinational bank to do software development, and one of my first tasks was to glean as much information as I could about one of the competitors' ATMs. My manager gave me a letter (on company letterhead) to carry with his contact info that described what I was doing, in the event security stopped me for spending an inordinate amount of time going through as many screens on the ATM as I could and writing down things about the UI. (Spoiler alert: nobody stopped me.)

I'm not an expert in airport security at all, but I'd bet dollars to donuts that anybody doing physical pen testing of that type will carry documentation describing who they are, who they work for, and what they are doing.