Final, final conclusion: if a computer is networked, consider it and the data on it to be semi-public. Make decisions about what to do and store on that computer with that assumption in mind.
Final, final, final conclusion: Interacting with a computer makes it networked even if you're not intentionally using traditional networking technologies (TEMPEST attacks, arbitrary code execution through direct user input, etc).
Physical access has always been game over. Having a networked computer means your threat model is literally everyone on the planet, which is a much bigger problem than keeping people from physically getting access.
Final, final, final, final conclusion: due to the complexity of computers, the only reliable way to achieve a moderate security in a system is to prevent it from being powered on.
