A one-range locator does not output a value for Ref_ID when the Feature ID is mapped to a valid unique ID field and the option to output Ref_ID is enabled.
Set the locator field mapping for 'Additional Field' to the ObjectID or another unique field. In the locator output settings, enable the option 'Write user-specified additional output field.' The geocoded output contains the additional field ('User_fld') with the designated field value for matched addresses, and blank for unmatched addresses.