When a membership is closed, the system will check to see if that member account was linked to any other accounts as a joint owner, beneficiary, IRA beneficiary, or additional signer.  If so, a non-member record will be created automatically and the links will be to that non-member record. 
NOTE TO NEWLY-CONVERTED CUs:  During the original conversion of your joint owner data to the new Secondary Names file, some of your joint owner links may have been made to closed member files, in cases where the system could not find either a MASTER or non-member record to link to.  Memberships closed from this point on will always create a non-member record.  In cases where both a closed AND a non-member record exist for the same SSN/TIN, the system will always use the non-member record for the link.