What might indicate that a METAR report has been modified?

The presence of the COR modifier in a METAR report signals that the report has been modified. This modifier is used to indicate a correction to a previous METAR that was issued for the same observation period. The correction could pertain to an error in the previous report, such as incorrect weather phenomena, visibility, or wind information. This is crucial for aviators and meteorologists as it provides an updated and accurate representation of the weather conditions at an aerodrome.

While additional forecasts, the AUTO term, and longer formats might be present in some METAR reports, they do not specifically indicate that the report has been modified. Additional forecasts could involve forecast information that goes beyond the regular observations, AUTO indicates that the report was generated automatically without human verification, and a longer format doesn't inherently suggest modification of the report itself. Therefore, the COR modifier is the clear indicator that signifies a report has undergone a modification.