[umami]

My understanding is that if you do not supply the key they can confiscate and destroy the media, though it will obviously vary from country to country.

The idea behind the hidden volume is that you can provide a key that works for the volume (and hopefully satisfies whoever is inspecting your data) while keeping your information private.

[umami]

Reading the link supplied by patio11 above (recommended), I wonder if using a hidden volume would constitute an obstruction of customs investigation.

"Don’t Obstruct an Agent’s Investigation

Once it’s clear that a border agent is going to search your device or other possessions, don’t take any steps to destroy data or otherwise obstruct that process. Like lying, knowingly interfering with a border agent’s investigation is a serious crime."

https://www.eff.org/wp/defending-privacy-us-border-guide-tra...