It doesn't respond to clicks or right clicks. I don't recall when it first appeared.
I'm running 10.11.6 El Capitan.
LibreOffice update icon.
Googling the image led me to MacRumours Forums
It is the LibreOffice update dialog box that triggers this. Clicking that icon opens the LibreOffice update dialog box, but it does not put it in front of other applications. That is what is leading to everyone's confusion about this icon.
To verify, I closed the update dialog, put LibreOffice in front of my other windows, and clicked the icon. The update dialog appears again.