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...