Earlier quoted context omitted.
Further in the article, Bloomberg mentions a chip, smaller than a pencil tip, sandwiched inside the PCB itself, under other traces. That seems bananas difficult to pull off, but could x-ray inspection find such a thing if it existed?
I specifically mention non-automatic inspection as automatic one may well not have the "paranoid mode," unlike a human who specifically told to go over every individual square millimetre.
Even at 1,000, I don't see how humans could reliably detect this.