PROBLEM
When opening the Geocoding toolbar in ArcMap and there are default locators set in the DefaultLocators.xml file, these should be listed in the toolbar. However, ArcMap sometimes opens and only lists <Add Address Locator...> on the Manage Address Locators drop-down on the Geocoding toolbar, even though the DefaultLocators.xml file exists and lists other locators.
This can occur when the DefaultLocators.xml has become corrupted.
C:\Program Files (x86)\ArcGIS\Desktop10.x\Locators
This should allow editing the new DefaultLocators.xml and successfully adding locators to the list.
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