Invisible Matters - Nickname1 in Use but Cannot See Matter

SUGGESTED

Timeslips is telling us that a NIckname1 (61390-00019) is already in use, but 61390-00019 does not appear on any open, closed, or inactive lists.  It is somehow invisible. 

Has anyone ever encountered this and know how to find the hidden matter?

In the meantime we added some text to the matter "61390-00019ARN" but would like to be able to use 61390-00019 as it is cleaner and less confusing for the client.