Google's security expert Michał Zalewski (lcamtuf) explored this subject in his book "Silence on the Wire" (2005). He even described how to build a simple devic

Google's security expert Michał Zalewski (lcamtuf) explored this subject in his book "Silence on the Wire" (2005). He even described how to build a simple device to read these LEDs. It's a great piece of the writing (much like the whole book). The chapter is available for free: https://www.nostarch.com/download/silence_ch05.pdf https://www.nostarch.com/download/silence_ch05.pdf