The first thing to look at is if the backup application is configured to encrypt the data or not. If you do have it enabled, then you have no worries.
The second thing I would do is to make a backup to another drive and simply look at what data you can see by browsing the drive with a Windows or Linux PC.
If the backups are contained in a proprietary binary file, I would doubt that most criminals would take the time to install/configure an instance of BackupExec just to possibly view your data. Assuming that the theft was a random act, and not a targeted attack, then there may be some security-through-obscurity.
Marketing by Symantec:
"Organizations are increasingly focusing on the security of the data they are backing up or moving off-site for long-term protection or disaster recovery purposes.
Symantec Backup Exec 11d provides 128-bit and 256-bit AES encryption to protect data when it is being backed up. Data can either be encrypted at the client-side to ensure protection when moving across a corporate network, or when copied from a disk-based backup to tape media."