Some antique safes were fitted with a binary chemical theft deterrent device as seen below.
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This generally consists of 2 glass vials of one containing sodium hypochlorite and the other nitro methane.
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The vials are housed in a metal casing mounted on the back or inside of the safe door in line with the combination dial.
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The idea is that if a thief were to drill out the dial a mechanism would trigger another device that would break the two glass vials thus causing a reaction producing chloropicrin gas a very effective respiratory irritant. Interesting and I would imagine effective deterrent.
This obviously is not your everyday incident, but the chance exists. This is simply one of those things that you file away for possible use at a later time.
Think safety and train often... CB