What could cause a Gemcrafter to stay inside his workshop for a long time and eventually die, while still being “undiscovered”

My gemcrafter locked himself inside of his workshop for a suspiciously long time. Eventually, he became "missing" and his yet-to-be-discovered corpse was releasing clouds of miasma. There are plenty of factors that could have caused this, but i will trt to add the ones i think have caused this:

He was working with Rough Onyx. (but never actually got started)
He was not assigned to any burrows.
He was "content".
He had an assigned bedroom.
He was NOT in any strange mood.
The fort was not in lack of food, water or booze.

Note 1:
I recently discovered his corpse (although it was visible that it was still in the gem workshop. The fact that it was "dehydrated" probably explains the miasma). So, why would my r gemcrafter isolate himself inside of his workshop?

Note 2:

His workshop layout:

WWWWWWW
W...ppW
W...pp
W...ppW
WWWWWWW

p = gem stockpile (was full that time)
W = Wall
. = workshop tile

Best Answer

When setting up a workshop, Dwarf Fortress shows you which tiles are impassible.

If you haven't installed a tileset, the default is to use a light green X for passable tiles, and a dark green X for impassible tiles. (If you are using a tileset, remember that the middle square of every workshop is passable, so when designating a workshop, the passable tiles are those that look like the center tile.)

In the case of the gem workshop, the layout is:

x x x
x x x
x x x

So all three tiles on the right of the workshop are impassible, which explains why your dwarf was unable to exit.

While the layout differs for each type of workshop, once you understand how Dwarf Fortress indicates impassible tiles, you can see when setting up a workshop where a dwarf can enter or leave the workshop, and where they can't, so you can easily look up in-game which tiles are impassible, and avoid setting up a workshop that a dwarf can't escape from, or setting up a workshop with inefficient stockpile locations (e.g. a stockpile to the right of a gem workshop is less efficient, because the dwarf has to walk around the impassible tiles to get to the stockpile).