I have also been struggling with this potentially useful shortcut because it did not seem obviously how to not have anything selected. So in "full view" mode you hit TAB and you end up inserting a tab break rather than getting the tools back. It would appear that the best/most obvious way to "not have an object selected" when you have no tool bars it to click an area outside the frame, sometimes referred to the slug area. Pressing TAB then works as a toggle UI as expected.