He had been injured by a goblin ambush the previous season, and then went to the fort on his own two feet (yes, he had both); disabled all jobs off doctors and forgot about it. Now I just noticed my pop had one less, Mr. Adon can't be located in the [ u ]nits list, be it living or dead, I haven't seen his corpse anywhere, and there are no mysterious clouds of miasma floating around.
Wait for it to say he's been missing for a week, or if you've already had the announcement, make a slab and engrave it at the Crafts Workshop, when you place it you can examine it (I can't remember if you examine it using k, t or q) and determine the exact cause of death.
That's just it, I never got any message, and he isn't in any list - citizens OR deceased. I just happened to notice, in mid-summer, that population number went down by 1; it's now the start of winter and he's just vanished. He's probably on the map somewhere, but beats me where; I spent the rest of summer and autumn navigating around, looking for miasma clouds, and bupkis.
I'll have to remember to open the game log this evening, maybe I'll find a clue; this should be fun, since my game log hasn't been cleaned in months and is bound to be in the multiple digit MBs.
[edit] And I'm sure I didn't mistook him for another dwarf; I got his name from the blood splatter he left when he was bashed around by goblins: it is definitely Adon. There is just one other Adon, a hammerdwarf who's in my military and who's hale and hearty. This Adon, the clothier, has vanished.
Oh yeah, in the meantime I also pincushioned a FB, bringing this fort's FB killcount to 11.