Uh, he stopped coming to work?
"He hasn't come back to his desk" might be a better translation.
He's not showing up to the office but hasn't had to endure the shame of being fired, but they aren't addressing exactly what happened.
Also "he never returned" has a different connotation in Japanese than in English. In English, never means never: bro up an vanished and never returned to his desk. In Japanese "he never returned to the office", especially with a time skip in narration, means "he didn't return to the office during the time skip" - which could mean truly never, ever or might just mean 'for a long time'.
about the not showing up:
Work culture in Japan is very weird. Some companies people will just stop showing up to work but won't be fired - in some cases their pay continues. I can't explain it very well but the same sort of moral socialization that stops you from just grabbing all of the free pens at work stops people from abusing the system.
It has changed a good deal since the 90s or ever since the 00s, but especially in the 70s when you got hired in Japan at a Japanese company as a corporate drone, both employer and employee expected that this relationship would last until you were no longer able to work, the company failed, or the employee massively fucked up such that firing was the only option.
Now career change isn't as rare as it used to be, but especially a lot of larger companies are slow to change.
Plus the
existence of the lethal Japanese cold work culture that idolizes working hard until you die or burn out, its not impossible that you will just not show up for a while. So its entirely possible bro had a mental breakdown and doesn't show back to work for a while. He also might have worked from home, or requested a quiet transfer to another department or office; he might have asked for a overseas reassignment or even to be transferred to another company.
Also its not like you just peace out with zero consequenes. He will be subject to various subtle forms of subtle punishment unless he has a very good excuse; at bare minimum his career trajectory will take a hit.
My friend who worked in Japan said a related company (literally, it was run by a "branch" family of the company president) had a dude who had offended company leadership in some way, and leadership response was to tell his division head he wasn't to be given any work, they even took his computer. Bro would show up, sit at any empty desk for 10 hours each day, and go home. (it was extremely fucked up, dude figured since he wasn't doing anything he'd at least take charge of the breakroom coffee pot; they made some new hire come in early to make coffee just so this dude would have nothing productive to do all day). I guess the guy had kids in school or something just had to put up with it.