Proximity to the power grid would be the main clincher, IMO, but what
about skywave propagation? That stuff could go around the world at QRP
levels!

No single signal will be heard by skywave; the power levels are too low.

Here's the math:

Power level: -50 dBm/Hz
Corrected to 3000 Hz: add 35 dB
Coupler loss: -10 dB
Powerline antenna gain: -10 dB
Path loss VOACAP: -110 dB

That gives a received signal level of -145 dBm/3 kHz, well below the ambient
noise at any HF station.

And millions of them may not make as much difference as we might think. They
way these systems work, they share bandwidth between many users, so the signals
are time muliplexed, so only one signal on a particular power line is active at
a time. 100 shared signals have the same peak field strength as 1 signal, for
the most part. So there may be 10,000 simultaneous emitters in a metro area.
That will add 40 dB, bringing it just above the band noise -- interesting, but
not conclusive.

The problem is local, and our concerns should be focused there.

73,
Ed Hare, W1RFI