Auto unlock to boot, ie autonomous booting... not unlocking of non system drives, but unlocking of the system drive in order to boot without requiring user intervention (i.e. entering the key).
Windows system passwords are laughable, the encryption is extremely poor by modern standards (no salts etc), and if certain network services are running (e.g. smb - running by default) you can login using the hash even without knowing the plaintext password.
If the system can boot autonomously, you can use specialised hardware to extract the contents of memory, which will include the password hash.
Even if you don't have access to such hardware, you can probably plug the machine into a small isolated network and try to attack it that way... If the system is fully patched you just keep the box turned off and wait for new exploits to come out as it's not going to patch itself without a working internet connection.