As I understand the policy (DoD employee), if a US traveller is carrying classified material in a designated, sealed courier container, and holding a courier letter from their workplace security office, then CBP is not supposed to open the container. They can call the appropriate home office to verify the identity of the traveller and validity of their courier letter.

I've never had to carry a classified "live" device like a laptop or classified phone while travelling. But I would think rules would require those to also be in a secured container while in such a public setting, I know we're not even supposed to take a classified laptop out of the secured part of our building. Either way, I would imagine the courier status exception applies.

Government-owned electronic equipment other than classified courier material IS subject to CBP search. Since CBP is legitimate federal law enforcement, our guidance is that we are expected to cooperate with such requests while on official travel with government-owned equipment -- it's the US government searching its own property, after all.