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Might have been audio? https://phys.org/news/2018-02-power-grid-fluctuations-hidden-audio.amp https://phys.org/news/2018-02-power-grid-fluctuations-hidden...

• 3036e4 1y ago
  Sibling comment and article mentioning "Electric network frequency (ENF)" lead to further reading mentioning both audio and visual fluctuations being used.
  • _neil 1y ago
    Ah you’re right. Pretty cool.