Now all you need to do is get a light sensor within line of sight of the LEDs, and away you go.
A Faraday cage would not let light escape. That is the definition.
How does a Faraday cage block visible light?
It’s a poor word choice I believe, a faraday shield also blocks light, while a cage can be seen through.
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.