Many tech companies add a link to their careers page in browser consoles. I'm not sure if that's "secret" or "cool" enough. (Facebook puts in a warning to prevent "XSS" attacks.)
AirBnB did this at least 3-4 years ago? If you watched the network traffic their HTTP headers included the email address for an engineering manager there. I stumbled on this while debugging something I wanted to file a bug report for. I didn't end up interviewing, but I remember thinking it was really cool at the time. I wonder how many applicants they got through that.