Reminds me of the phenomenon of “cross-cueing” in split-brain patients. With the main connection between hemispheres severed, information can still take the long way round: one hemisphere produces a bodily cue (eg hand tapping) that the other picks up through the senses.
There are literally dozens of ways to get around air gapping.
- LED lights on the machines, max bandwidth 100 kbps out, 18.2 kbps IN (yes you can pump data IN through an LED on a machine)
- Noise from the HDD, fans etc, max about 200 bits/second
- electromagnetic from cables, power surges from CPU load etc etc