What are the "Overlapping Squares" icon in status menu on login screen? On El Capitan's login screen the status menu (top right near time) shows an icon of two overlapping squares, one square is to the top left and looks behind the other. 
It doesn't show once I login.
What does this icon represe
Does the icon look like what is in this image? 

http://cdn.osxdaily.com/wp-content/uploads/2014/11/screen-sharing-messages-app-mac-os-x.jpg

If so it sounds like it would be related to Screen Sharing.