The answer is the text field of $:/temp/search. (You may also have found $:/temp/search/input, which is an auxiliary tiddler used for the keyboard-driven-input that handles the autocomplete functions of the search box, and often contains the same text.)

The easiest way to find this answer is to type a weird word in the search box that you won't find anywhere else (e.g., oobleckblargle), then open up Advanced Search and search for the same word on the System tab. The tiddler will appear in the list.