Go to https://secure.phabricator.com/ and run a search in the top search bar. This runs a search over open + closed documents.
Now go to any individual application (eg, https://secure.phabricator.com/w/) and run a search in the top search bar, scoped to "Search Current Application." This defaults to searching only over open documents.
I find this inconsistency to be confusing, and would prefer if the default was to always search only open documents.