I hadn't played my tribe for a very long time until I decided to go back to it yesterday. At the time I unpaused the game, my oldest villager (in his 70s) was one of the original 6 (5 villagers + baby) - he had been a young child when I started this tribe.
I left the game running during the night (didn't check on the tribe until morning), and, as I had expected, in the years that had passed in game, several villagers (6 or so) had died of old age. When I later checked the mausoleum, everyone was there except for that one villager.
I didn't get any event that might explain his disappearance (like the one with the box at sea, for instance, that would result in a death but the villager wouldn't be in the mausoleum), and I didn't interfere with the villagers burying the dead either, so I just can't understand what happened to him.

The villagers decompose after a while, sadly.

What box at sea?

Oh, I know.

BTW: you can pick up people while they carry villagers to the graveyard and the skeletons will teleport to the graveyard and turn into ghosts, instead of waisting your (and their) time.