My best guess is that something on the unencrypted Mac Pro was referencing files on the encrypted drive. I'm not sure what kind of metadata macOS stores or what

My best guess is that something on the unencrypted Mac Pro was referencing files on the encrypted drive. I'm not sure what kind of metadata macOS stores or what a third-party app would have, but it wouldn't be hard to imagine that something had hashes for files on the external drive.