I didn't notice it until you told me
![Smile](https://www.natmedtalk.com/images/smilies/icon_smile.gif)
. It's a built-in feature of vBulltin and must be there since the beginning.
I only noticed another sign at the first column, it is displayed when you participate in a thread. Both signs will pop up a window when you put your mouse over it, but with different information.