Quote from
Chris Scott:
Think what probably happens sometimes is that the METAR is broadcast before the system has tacked the TREND on to the end of it. When the omission comes to someone’s attention, the METAR has to be reissued as a “COR!”.
COR is automatically inserted into all METARs that are re-broadcast with a Trend appended. If for some reason the Trend doesn't get added the METAR should appear without it. Most online sources appear to strip out the COR anyway. Hope that helps.