A research group once, I shit you not, put varying CPU loads on a target airgapped computer and “transmitted” data encoded as fan noises going high and low. There have also been cases of getting GPG keys in lab settings by literally listening to the tiniest of noises made by the processor as it works! (I believe it’s coil whine? But I can’t seem to find the article explaining it) Information leakage is an absolute nightmare to try and completely prevent if your attacker is determined/skilled enough.