When I approach a security check in an airport, I often see some kinds of instructions in the front side to teach people what items I need to put into the trays.

It's one of the critical tasks that require our full collaborations to ensure the safety for all of us. So from the perspective of airport staffs, they need to teach first-time travelers and also remind experienced ones.
Hence the simplest idea is to show some flyers or guidances in the front side of the security checkpoint, and hoping people would read them.
Recently, in the Amsterdam airport, I noticed its tray design works amazingly.
The items you'll need to put are drawn inside the tray! It serves both as an instruction and a reminder at the right timing and right place. When you take one tray, it's right there to guide you.
(And plus it's twice big as the one in Tokyo!)
I guess the security checkpoint is the place where we could assume that people would come with the right mindset - to pass it, they will have to put the right thing into the tray to be examined (unless they want to get caught on purpose). Even for first-time travelers, they can get the picture while waiting in the line.
But not everyone pays attention to the instruction, no matter how well the illustrations are embedded, it still appears complex when it contains just some bold text, and we humans are lazy, we want simple stuff.
And here is the genius part of the placeholder design. All people will take at least one tray, that's the perfect chance to teach them, Not before, and certainly not after. They're guaranteed to see it. The placeholder provides such contextual guidance, that removes all unnecessary hurdles and assit us with zero cost.
I feel so fascinated about it.