Academic studies have demonstrated that adjacent air-gapped computers can communicate through temperature changes, with one heating its CPU and the other detecting the change.

“you know like there there are studies like and this is mostly academic but um where you can have two computers next to each other that are air gapped and they're still able to communicate with each other because they have temperature sensors and they're able to like one of them is able to run their cpu really hot and then the other one can actually like detect the temperature change and then they can that actually gives them mechanism to communicate”