Maybe some kind of NFC device that you could wear so that the computer would lock/unlock when you were near it ?
That would be perfect, but instead of wearing it I could have something implanted in my arm for instance; and I would definitely notice of somebody tried to steal the tag!
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---going further---
Then just encrypt the home folder's contents on a file-basis & sync it with a privately owned server. This improves the chances of data-recoverability on crashes or on filesystem corruption. Add a kill-switch toggle that erases all important files on the mac on 5 unsuccessful login attempts, and enables surveillance mode permanently using different tracking software (prey-project for example).