Actually security clearances do include an NDA. When I signed mine it contained an amusing clause, something to the effect of you will not share classified information until 70 years have passed or you die, whichever is _later_.
So if you live past 70, you could write classified information into your will?
Could the person who drafted that have been contemplating something like a Dead Man's Switch? Even if so, not sure how it would have much teeth in terms of consequences after you're dead.
Or some weird scenario where an individual technically dies but is then brought back to life?
Or maybe they secretly recruit zombies and only drafted one set of employment contracts.