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.

Modems 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.

With 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.