Must be 25 years ago now I was chatting with a family friend, my friends dad, and he told me roughly what it was that he does:

"If I have physical access to your computer, I can probably get whatever data you have off of it". It was the first time I had even heard of computer security as a concept.

I don't know precisely what methods he was using back then (it was the 90s) but he was dead serious.

The point he made has become more and more true over time: Whoever can touch the machine, you presume they have better access than the admin.