Public secret code....
You want a code or an instruction public available, but only in case of an emergency.
Here the code is written on paper and there are 3D printed around it. You can now clearly see when some one have assessed the code.
Give a friend your house, garage or boutique key and the sealed alarm code and he can only enters the place when he have broken the seal.
You could even seal a key in a 3D print.

Ohhh yes... You can use it for the next Grammy show ;-)

Print the box bottom and sides until it will print the top lid.
Pause print or chose change filament.
Put in label and black paper. The black paper prevents that someone can lite through the plastic an read the code or message written on the label.
Start the print again in slow speed and carefully hold down the label while the printer print a layer over it.