A security vault was breached without tripping the silent alarm, and the master key was still hanging on the wall inside the supervisor's office. The only way in was through a heavy steel door locked by a mechanical combination dial. The night watchman insisted he never fell asleep. Detective Cole pointed out a thin layer of dust on the dial's numbers 4, 7, and 9, while all other numbers were clean. What did this prove?
The safe combination did not include the numbers 4, 7, and 9.
A combination dial's correct digits would have fingerprints wiping away the dust, leaving those untouched numbers out of the combination.
About this riddle
Detective riddles are tiny mysteries. Look for the detail in the story that does not fit; it is usually the clue that solves the case.
Difficulty: Medium. More like this: Detective riddles with answers or try Truth & Lie Island for a different kind of puzzle.
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