Right. My bad. In theory though the question still stands. What happens if I
a) remove your entire disk, and
b) replace the encrypted disk with an unencrypted disk (which still prompts for the “LUKS” password and then politely transmits it to the hacker who can now decrypt your entire disk)?
Of course that won’t be subtle. Once the computer boots, you will be able to see that your disk is no longer there (unless the attacker is more clever than the blunt attack described above).
An obvious way to counter that is for the firmware to be configured to know that the disk must be encrypted and refuse even to boot if the disk is not encrypted.