Good judgment is the most important quality for anybody that is expected to arrest people at gunpoint. You make the wrong decision, and things go sideways. Give the people with good judgment lots of training and education, and they will figure things out.
One way to handle a situation like this in the U.S.,
"Excuse me, miss? What exactly is that thing on your shirt?"
"A circuit board with LEDs connected to a battery."
"I need to see that for a second."
"Sure."
"Why are you wearing this?"
"I can do anything I want. It's a free country."
"Well, some people might think you are wearing a bomb in an airport. That would seem to be your motive. Perhaps a 'statement' or practical joke or test of security to document on a blog or Youtube. The airlines are under no obligation to let you on an airplane wearing that. If you don't want to miss your flight, you should consider putting that thing where people won't see it."
"Are you arresting me? I haven't broken any laws!"
"You aren't under arrest, but since you won't be getting on any airplanes wearing that, you won't be allowed through security. If your intent is to scare people, you will be removed for trespassing and disorderly conduct. Your choice."
I've done airport details and arrested people there. There was a complaint phoned in to my supervisor while I brought the arrestee back to the station. After explaining my actions, the supervisors realized my conduct was low-key (no guns drawn, loud voices or even handcuffs), professional and absolutely correct, and the complainer (a ticket counter employee) had lied to airport police to get me in trouble. Simultaneously, my coworker was arresting (using yelling and physical force), a provacateur with a video camera that sued him for $2 million and settled the suit for 6 figures.
Finally, I was doing the Firearms Simulator during my qualification last year and one scenario had a guy in front of a school bus wearing a bunch of dynamite sticks and holding a button connected by a wire to the sticks. He is yelling about not wanting to go on because his girlfriend dumped him, etc. I'm standing there, gun aimed, when BOOM! He blows us all up while I vainly shoot my gun too late. I muttered, "I am the first U.S. law enforcement officer to be killed by a suicide bomber." Training us to shoot anybody who claims to have (or appears to have) a bomb has some significant repercussions, until we start seeing real suicide bombers in the U.S.