Once you do a thorough sweep and decide one device is clean all it takes is a small PCB revision and you have to start over. It becomes a very expensive exercis

Once you do a thorough sweep and decide one device is clean all it takes is a small PCB revision and you have to start over. It becomes a very expensive exercise after a point (time and money) that is reserved for the most critical of situations. The problem is a good hack doesn't need the most critical of situations to succeed. It can creep in through the most innocent cracks and climb its way up the chain. And some hardware hacks are beyond what a regular Xray PCB check would outline, like a modified piece of silicon in a chip. Xrays are great for more obvious changes, not necessarily for something that was inserted at the very start of the supply chain.