Not updating its timestamp livelink
That kind of timestamp comparison is actually the basis for incremental backups: backup only those things that have changed since the last backup.
Truecrypt volumes are container files that, in turn, contain in encrypted form a complete file system along with all the files and folders you choose to place in the Truecrypt volume.
Note: your container may still be dismounted in order to be backed up.
When the container is mounted Truecrypt locks it such that most other applications cannot actually access it.
Exposing it by updating the externally visible timestamp of the container - something that can be seen without needing the passphrase - actually boils down to a form of information leakage and is a potential security/privacy risk depending on how you're using Truecrypt. The volume doesn't get backed up because as we've now seen Truecrypt doesn't update the container's timestamp by default.