I enjoyed this episode! It is an interesting topic. I would like to hear about case studies. 

Somewhat related. At least I heard that some persons writing some technical documents, hide their "kilroy" in eg a drawing, like maybe tiny black text on a black line. Somewhat similar is hidden code, like I recall a Tektronix oscilloscope that if you entered a specific order of commands, a figure would play on the screen - I didn't find it out but I remember a colleague did. And some software has hidden games in the code. 