Shift-F10
As to it's usage, I've no idea. It's been brought up every once in a while, in a similar situation to your - accidentally opening it, no documentation at all. It seems that it displays some "engine level" info about objects in the active and adjacent cells, but I've no idea if it's of any use for modders, without the access to the engine itself.
Another undocumented shortuct I've found is Ctrl-F10. It loads one cell after another, probably for testing purposes? And I've found it just now, when I started trying out different combinations to try to find the Scene Graph one. Button mashing can be a pretty effective way of discovering undocumented features!