The idea
You already know how to keep an object.
Nobody needs a tutorial on house keys. You know not to leave them in the door, you know roughly where they are, and you notice within a day if they are gone.
Digital security asks for the opposite: constant vigilance about invisible things, in a system whose state you cannot observe. Whether your data is safe is, honestly, a matter of trust.
Putting a vault on a card returns the question to a form you can answer. Where is it? In the drawer. Who has touched it? Nobody. How would I know? Because it would be missing.
Why it works
Three properties of physical things.
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I
Absence is visible
A copied file leaves no gap. A missing card does. Physical loss announces itself.
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II
Reach is limited
One person can attempt one card at a time, in one place. Scale is not available to them.
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III
Custody is intuitive
Deciding where a card lives needs no security expertise, just the judgement you already use for documents and keys.
Where is it?
In the drawer.