Why does the Terminal app icon show an unread message? For the first time in roughly three years, the Terminal app icon in the dock shows that red new message overlay.



To the right, the same happens with Thunderbird which I am used to as I do have three new emails. But what does that new message overlay in Terminal app mean?

I have no new mail in /var/mail/$USER nor does Terminal show any message when started.
That badge indicates a "beep" that happened while the terminal was not in the foreground. 

To see it, you can send a beep command to your terminal. Type this command, hit return, then click to another window that is not a terminal window:

sleep 5 &amp;&amp; tput bel


Are you saying that it went away (as it should) if you view all open Terminal windows or tabs? Or is it stuck on? Or you were just wondering where it came from?