If you backed up the BKF files to a tape drive, you will not be able to search the tape for the file. As our catalog only knows about the BKF files on the tape. What you will need to do is figure out which bkf file then data you are looking for is on, then recovery those BKF files for the restore.
If you would like to be able to search the tapes for a specific set of files, you will need to use Duplicate jobs to duplicate the data from the backup to disk folder to the tape.