That is a good question.
I don't believe FV will encrypt what the OS considers to be unused space on the drive.
You probably know that files that have been deleted aren't actually removed from the drive, but their storage blocks are marked as unused/reusable. Over time, those blocks will be overwritten by other data, exactly why data retrieval becomes more difficult if a drive remains in continuous use. I believe FV ignores any data in this "unused" space.
As I understand it, FV encrypts OS accessible data currently indexed in the drive directories and new data as it gets written to the drive.
NOTE – when erasing a storage device, if available, click Security Options, use the slider to choose how many times to write over the erased data, then click OK. If this option is available, then there will be no data in need of encryption.
Secure erase options are available only for some types of storage devices. If the Security Options button is not available, Disk Utility cannot perform a secure erase on the storage device.
Writing over the data three times meets the U.S. Department of Energy standard for securely erasing magnetic media. Writing over the data seven times meets the U.S. DOD standard.