Well... who is "the player" in DF? Is we some omniscient godlike entity, or are we meant to be representing the expedition leader/mayor/king/whatever?
As a (I think) simple idea:
1) Each dwarf has a timer - might be a season, a year, whatever. When that timer reaches zero, they are "missing".
2) When a dwarf sees another dwarf, the seen dwarf's timer is reset, unless the observer is also missing.
3) The ranking noble has a slightly shorter timer (say, three seasons instead of a year), but it acts differently. When his timer runs out, the "ranking noble" role switches from him to some 2IC or something.
This system, unless there are some flaws I can't see, would simulate a "chain of reporting" to some extent - the king might not have ever seen Urist McRecruit, but McRecruit has seen/been seen by McCaptain recently, McCaptain has seen/been seen by McGeneral recently, and McGeneral has seen/been seen by the king recently. The only time masses of people go missing would be if they were all walled of somewhere - which kind of makes sense.
If the King falls down a hole and dies, alone, you don't know straight away. Dwarves would continue along, being seen and unseen. When the kings shorter timer runs out, the role of "ranking noble" switches to his heir or, if there is none, to the next-highest noble. If there are no appropriate nobles, then a "crisis leader" is elected, in the same way as the expedition leader or mayor is. This shorter timer means that (a) the ranking noble has to be kept in the loop, preferably somewhere central-ish, which makes sense, and (b) you never get a situation where the baron died, and you find out just in time for half your fort to go dark.
Going off on a wild tangent - and I don't expect this bit to be implemented, it's just idle musings - suppose there were multiple "seen by" groups? If the ranking noble idea were in, that would be group 0. This is the only one the player sees. When two missing hunters meet, they become group 1. If three other dwarves, walled and lost in the caverns meet, they become group 2.
This has one good effect: when those two hunters are out wandering, and one is found by civilization, they can go "Oh, hey, the fort! Yay! Hey, I'm safe now, but Hunter McMapreadfail is still out there! I saw him by the Forests of Infinite Violence!". Bam. Group 1 gets folded into group 0. The player can now go and find the missing McMapreadfail.
The other possibility would be that, after some time, the new missing group becomes a new entity (but not a new civilization). Once off-map travel is possible, if missing dwarves can wander off screen, they could spontaneously found a new group as they settle down. Finding a lost group of survivors from a fort you played a few (in-game) decades ago would be pretty awesome. Recruiting the son of your old legendary axedwarf, and kitting him out in his father's gear? Awesome.