There is always a way. A program could take control of the LEDs on the PC and export data by flashing them in certain patterns.
replyModems in the olden days used to have LEDs to show traffic directly attached to the TXD and RXD lines coming off the level shifter, and therefore directly showing the presence or absence of a bit stream on the serial port.
replyWith a suitably tightly focused telescope you could pick up the LED blinking and decode the data going over the port.
A Faraday cage would not let light escape. That is the definition.
replyPutting it beside a Tesla coil would work too, make more noise than the computer does and you'll bury the signal.
replyOf course, now you've got a Tesla coil in your data centre, which will bring you a whole new set of Interesting Problems to cope with.
Yes, Faraday shielding is one of many considerations when building a SCIF or other EMSEC zone.
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