Mr. Feynman famously found you could lift the combo off a safe [with the a-bomb's secrets] when it was empty. When he alerted the Colonel not to leave his safe open, the response was to:
send a note around to everyone in the plant which said, “During his last visit, was Mr. Feyman at any time in your office, near your office, or a walking through your office?” Some people answered yes; others said no. The ones who said yes got another note: “Please change the combination of your safe.
That was his solution. _I_ was the danger!”