If a posting in whisper mode is started but the page is reloaded by some means, it will revert to a non-whisper.
This can cause unintentional posting of information meant to be communicated only within a team since the whisper icon can be fairly subtle.
You can also look at the text in the preview, if it is all italic and grey then it is a whisper.
In the case above that @supermathie is referring to it was just an image which is how it slipped through.
That helps on desktop but is much more subtle on mobile.
@maja fixed this a few days ago.
https://github.com/discourse/discourse/pull/5417