They send all typed into editor input to codepen.dev almost immediately (you would see in 1-2 sec after you typed your secret that it appears in respective Network/Response tab) even before one saved it. I tested this with a unique marker: after typing it into index.html, CodePen ran a build with "save:false", and the marker then appeared verbatim in the HTML served from the generated "*.codepen.dev preview". Thus, if you ever entered some secrets in there by mistake consider them compromized ev...