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If it's an electronic safe, maybe rig a secret button that turns it off? I think all the safes that I've seen that would qualify as Ft Knox have a way to get in even without a battery though. I dunno... Then there's that whole "can't prove a negative" thing... If some thug is convinced you can open it, about the only way to prove to them that they're wrong is to take the bullet without opening it (and even then they'll probably just think that you could've opened it, but didn't really think you'd get shot or something).