The Index of Unconfigured Graphics is listed as Missing Graphics under Miscellaneous Indices in the Table of Contents, in the index pane. Clicking on it will bring up a list of all the document hotspots that are supposed to link to other documents, but don't. Clicking on anything on the list will bring up that document, and highlight its broken link.
Selecting the same thing in Treeview format would look like this:
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