Have the laptop continually monitor the physical environment - if anything abruptly changes, system automatically locks. If a decryption key is not provided after X seconds, force a reboot.

Something like bluetooth beacon paired to discreet BLE jewelry, WiFi signals -if the paired connection suddenly disappears, newly inserted human input devices (mouse jugglers to disable screensavers), unregistered face-detected, minimal gryoscope add-on looking for sudden velocity change, etc

All of these carry significant usability trade-offs, so probably only worthwhile if you are running the Silk Road and actively hiring hitmen.