Clear is probably a requirement so that the guards can see inside and be reasonably sure it hasn't been tampered with, for example, to communicate back to the outside world.
Unless they have a particularly good IF isolation amplifier, you can use it to communicate back to the outside world. Someone would have to be listening, of course, with a sensitive receiver and directional antenna.
You can probably turn the dial and even get some FSK going on.
Anything else with sufficient internal voids to conceal contraband, too, I should think. It's a lot easier than taking every such item to pieces during every inspection.