But if we start to systematically check for it, it should be easy to discover via hardware debugging. Find the JTAG interface on the hard disk controller (or whatever debugging interface the specific processor uses), dump the firmware and compare it to firmware dumps from other hard drives of the same model. I don't see how they could fool that process (given that you have a clean machine to read out the firmware).
Of course to be thorough you would have to check pretty much the firmware of every component of the computer.