I suspect you schedule a delete volhist type=dbbackup todate=today-x-days
command.
If x-days in del volhist is
    LESS than the  SET DRMDBBACKUPEXPIREDAYS value plus the time that it takes
to get the tape from vaultretrieve to onsiteretrieve
then
   the consequence is that you loose your track of DB backups in DRM and you
have uninventoried tapes in the vault.

So, I suggest you eliminate the delete volhist schedule. This is ok unless you
have two sets od DB backups, local and vault. If this is the case then ensure
del volhist deletes backups older than the drmbackupexpiredays + time....

Regards, Giulio John Tarella  IBM Italia