Most of these signals are not so secret, as demonstrated at this DEFCON presentation:
<a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=ZuNOD3XWp4A" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=ZuNOD3XWp4A</a>