There's a comic out right now positing that a sufficiently intelligent AI with appropriate access could use imperceptible (to us) vibrations from mechanical computing parts like spinning rust HDD's etc.
It's a throwaway mechanic in the comic, but it seems plausible.
It's a throwaway mechanic in the comic, but it seems plausible.
In certain places the power companies are/were passing time information throughout the whole grid - https://www.nist.gov/publications/time-and-frequency-electri...