A missing customer/member record coming from your core import file is NOT marked as a Past Relationship. It is actually flagged for deletion from Core iQ at the time that it is deleted from your import file. To be sure that all of your customer/member data is persisting into Past Relationship status, please be sure that your core extract file includes customers/members that no longer have accounts. These people will simply be marked as a 'Past Relationship' and will continue to be usable in schedules and campaigns within Core iQ.
If you no longer provide us with ANY data for a customer/member, we will not persist the Core iQ record for that individual any longer than the time set in your Bank & Core Import Setup area under Days Before Committing Core Deletion. At that point, we will delete the record completely from Core iQ. However, prior to that point, we will also have potentially deleted all contact information for the customer/member, as it was not supplied in your core extract.