If we were to, say, deconstruct the relevant processor and image it under an electron microscope, how obvious would these mechanisms be? I imagine intelligence

If we were to, say, deconstruct the relevant processor and image it under an electron microscope, how obvious would these mechanisms be? I imagine intelligence agencies in countries which are not huge fans of the US would have every incentive to do this. They might not be able to figure out the mechanism (i.e. access the backdoor), but they would obviously try to work on processors that aren't compromised, which might be noticeable (Loongson?) or they could simply reveal this knowledge to embarrass the United States, and it would certainly cause some political backlash if confirmed. Basically, "I can't tell" is a much weaker guarantee than "nobody can tell" and ensuring this latter statement is going to be difficult even for the NSA, I would presume. Also it is not entirely clear why, if such vulnerabilities do exist, why they would not have ever been used for anything, such as Stuxnet, because if they cannot be used for anything then what could be the point of having them? We're all familiar with the argument that the government will not disclose a hidden surveillance mechanism to catch a target that isn't "worth it" relative to the loss of secrecy, but if a suspected (however weak those suspicions may be) Iranian nuclear weapons program isn't worth it, then what, precisely, is? Furthermore, is it politically feasible? The government has already seen leaks by the likes of Snowden and Manning, and this development would require even more time and effort and cooperation across teams of people who may not be entirely supportive of the far-ranging surveillance this would imply. It's one thing to backdoor a server, but to modify the architecture of the most advanced consumer products in the world is probably not going to be easy. So I don't think it's impossible, but even the NSA is finite. They probably have better ways to spend their time.