>How many border protection agents look into your /etc/fstab and know fdisk, mount and cryptsetup?
They don't need to. Microsoft and other companies have been providing software on bootable CDs and USB keys to do quick 'security' audits of PCs for years to various police forces. I'd be extremely surprised if detecting encrypted/hidden partitions wasn't a common feature.
it would be very useful to know exactly what tools and techniques the border agents are using.
Is there software that detect HDDs that are disabled via BIOS? Do the border agents reset the BIOS? Are they looking at individual files and encrypted archives?
They don't need to. Microsoft and other companies have been providing software on bootable CDs and USB keys to do quick 'security' audits of PCs for years to various police forces. I'd be extremely surprised if detecting encrypted/hidden partitions wasn't a common feature.