Tracking someone through landlines has been a Thing for many years now. Ever hear of a "lock and trace"? You can SORT OF do the same thing for power, by embedding a signal in a given substation. It's nontrivial, and it's horribly complicated, but it IS feasable. As for the "hum" thing, that's just standard TEMPEST, been a Thing now for going on thirty years, where you can fingerprint electronics via EM signatures and you can read those EM signatures via physical phenomena including audio hums and induced currents in surrounding circuits. This is why the LASER mike was actually developed, not for actual sounds (standard shotgun mikes do wonders there, because the glass reresonates sound just fine), but to get a good frequency signature on TEMPEST EM leakage. So, in sum, they're not specifically taking a van out and following lines to see what location an interviewee is at, but a lot of that is that they don't really need to because they can get all the information they need through older technologies that approximate the capabilities
HUGE problem with this theory.
The power grid operates on incredibly tight tolerances with regard to frequency. Additionally, within that margin (which is the same, everywhere, within a certain grid...and by grid, I mean, like "The United States" or "Great Britain") there is a small degree of variation that is the same for that grid and all that are built using the same equipment...which is a significantly humongous population.
Imagine a metropolitan area like, say, San Antonio. San Antonio has several power stations that service its region. Each generation turbine produces what's known as "three-phase power," which is kind of like TDMA for AC electricity. Those three phases get broken out and separated into three outputs that then go into a substation and transformers, then out on the grid. The three phases equally and perfectly distribute around the 360-degree rotation of the "exciter," which is basically the generator's key component. If that distribution gets out of whack, power spikes in a really nasty way, and copper vaporizes fast enough that it's actually a detonation.
But I digress. The point is this: AC power is a waveform, oscillating at 60 Hz. It cannot vary much at all...because within the same grid, everything is interconnected. Every generator is in sync, or has a syncrophasor to re-sync the power coming from it before it hits the grid. Otherwise, you get some power from A and some from B, with waveforms that are out of sync...and the frequency changes in both rate and amplitude, and shit blows up. (Including generators themselves...the "Aurora Vulnerability" that DoE is so batshit scared of is essentially a manifestation of this at the generator itself.)
So...I've been trying to think of how there could possibly be enough variation to fingerprint someone based on the hum caused by that 60Hz frequency noise. I've been in transmission control centers where they monitor, regulate and occasionally wet themselves over frequency shifts, and I've seen that the amount of variation needed to cause sheer panic is shockingly low..and it rarely ever happens for even a second. And those tolerances have been the same everywhere I've gone.
So no, it's not at all like TEMPEST. Because if it were, it'd be the equivalent of being able to figure which monitor you were looking at by EM emissions...when all the monitors in the country show the exact same thing.