At least twice in the last decade, he's been unemployed so long his benefits ran out. Perhaps the label in that case isn't "retired," but he wasn't counted as unemployed, either.
The BLS tracks six measures of unemployment rate via survey called U1-U6. U3 is the official unemployment rate, but they report the others as well[1].
For U3, you are considered unemployed if you do not have a job, have actively looked for work in the prior 4 weeks, and are currently available for work. U4 is that plus anyone who wants a job, but gave up looking because they don't think they will find one. U6 adds in people who only have part-time work but want full-time work.
[1] https://en.wikipedia.org/wiki/Unemployment#United_States_Bur...
They basically survey 60,000 people and ask a specific set of questions to figure out which "bucket" people are in.
https://www.bls.gov/cps/cps_htgm.htm#unemployed