I am guessing that you recently updated to DP 8
The Upgrade involves the use of an entirely new IDB, which make use of a newer version of Detail Catalog Binary Files (DCBF). Each of these files holds all information of what is on a corresponding media
Yhe upgrade to DP 8 does not convert the older (DP 7 and below) DCBF file to the new format. Instead they run side-by-side, with the ability to Restore using the older version of the DCBF. As the sessions on the media become unprotected, and the media is reused, you will see this message in your event log, with the number of Old Detail Catalog Binary Files getting smaller, until they reach '0'
So, the message is just telling you that you have no more Old Detail Catalog Binary Files