Interesting that this thread came up. Just recently, on another forum I'm a member of, this same discussion popped up. The OP, over there, also wanted a safe room. His original idea ala-The-Munsters was wanting a stair case that raised up. lol
One suggestion was this
... but the price tag was a bit too much for him, even with the cool factor.
Two of the better suggestions were the following links...
Out of sight... out of mind. Sure, someone with an axe (like stated above) could hack their way in. However, if you do it right, they'll never know it's there.
Maybe the above info will give you some ideas, if not a plan, to use in your own remodel/build. Good luck.