Why does the Terminal app icon show an unread message?
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?