Tapping out a signal that way would be troublesome. Computing equipment might sense power sags, but would be oblivious to any signal frequencies riding on the power wires, and would have no way of modulating same. That is, not without purpose built equipment, such as ethernet over power lines. And causing a sag to happen would be instantly seen as a bad, bad bug.
I have a degree in signal processing. It's been a long time, but, having seen switches capable of sensing distinctions at -60dB, well, I have no idea any more what these guys can do now, so I asked. It would take a trigger string or spread spectrum key, but with so many devices that's not likely. The liability would be that once known, it could easily be canceled. The problem for the listener would be detecting and untangling the multiple identifiers amid so much noise.
My point in asking the question was to point out how unlikely it is that if the Chinese are that motivated to penetrate our systems, then we just should never buy the stuff. It can't be that much cheaper and what with the requirements of Mil-Spec documentation, they probably cheat on that too. Not that I think all that paperwork accomplishes diddly, but it is unfair to the domestic supplier. The benefit to us of having our auditors on the ground there was clearly illusory.