I use Linux. I'm convinced that if I put a small Windows partition up (or another Linux install) and make grub boot into it automatically (with little delay) no one would ever notice. Does any one know if they check for multiple partitions at all? And Android can have multiple users, can you set up a new user and boot into that one automatically?
well, with some experience, border guards will learn to spot a non-ntfs partition, that windows dutyfully reports as unformated/blablah space, so that will eventually be caught, dont rely on this to actually protect you, its more of a sleigh of hand that may be easily spotted by the right guard.
Another hack would be one's own BIOS, that lies to Windows saying "This disk is 100 GB", but given the correct unlock signal, will admit to the OS "this disk is 500 GB big".