... and sound proof too...
In the 1980s I worked on a computer that had a whistling power supply. The ferrites would squeak at different amplitude/frequency depending on the load. You could run a for(...) loop and count the iterations by listening to the warbling of the power supply.
... and light proof too... since you can get info out via the screen.
... and heat proof too... since you can get info in/out via temperature sensors and CPU load causing increased temperature.
... and operate from a battery... since you could also get data in/out by modulating power draw or monitoring voltage on the power cables.
... and no doubt tens of other mechanisms.
Biting the hand that feeds IT
