Re: This is a Real national security problem. Think hard, Brits!
You are taking a criminal law approach to a national security problem. Admittedly, that's because national security has been so stretched over the years that as I said, people demand standards now even for national security problems.
To take what you said in a more negative light, they have actually found leaks but CHOSE to believe they are innocent ones and at least as important, the Chinese government don't already know about at least some of the leaks.
You also have to be sure the source code is exactly the same as those that go into every machine, check all the patches they'll inevitably publish ... etc, etc. And even then it is a contest between your ability to find the holes and their ability to hide them.
Once you have committed, what ARE you really going to do if you start finding the holes then? Do you think reversing would be cheap or even possible?