Viewing Mains - gone

Every once in a while, my Mains disappear (I probably toggle out of them somehow), and I can't get them back.  I usually see them in with my buses in the Console view, or in the Inspector, but they are in neither right now.  What am I doing to hide them, and how do I get them back?
 
Thanks

You can also select the divider with the mouse and slide it over to reveal the Mains or the Buss pane or both.