While I also doubt that this is possible today, I am sure the NSA is looking at placing the respective sensors. Then we will have to do "analog routing" and mix in mains hum form several places to obscure where and when things have been recorded. Maybe we should start to offer recordings of local grid noise. Would not be that difficult to do.
It's not even that complicated.
Many power lines have optical fiber strung in the middle of them, it's called optical power ground wire (OPGW) (scroll down a bit). That fiber is used as Internet backbone, as telecom voice, and as diagnostic for when there are power grid problems. If a line goes down then they can use an OTDR to determine the distance to the break instead of having to hunt for it.
All that they'd have to do would be to put devices at termination points and use dark strands. Sure, the equipment to transceive on single-mode fiber at those distances would be pricey, but it's completely within the technology that we have right now.
The sooner you fall behind, the more time you have to catch up.