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.
You don't need an AI to come up with remote sensing or air gap traversal capabilities though.
Note for example TEMPEST surveillance, or using a distant laser to pickup speech in a room based on window vibrations. Air-gap traversal is easily done by exploiting human weaknesses (e.g. curiousity to pick up a USB drive to see what's on it), and was successfully done by Stuxnet.
Indeed, there are lots of methods, but i was specifically thinking of the possibility of a method an isolated AI might feasibly figure out with only the tools it has easily available to it.
But as someone said earlier, the real interesting part is when/if they start figuring out novel concepts we as humans haven't even considered.
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...