My friend Dan wrote:
The word is “unsub.” Spellcheck doesn’t like it.
1) Google: Unsubscribe, as in cancel a subscription.
2) Everyone else: Unknown subject of an investigation. Used mainly by US TV crime shows.
Hmm, I don’t watch the crime shows, but let’s check it out.
1. Unsubscribe.
‘you won’t be spammed and you can unsub if you change your mind’
Origin
1990s: abbreviation.
2. US, informal
(in police use) a person of unknown identity who is the subject of a criminal investigation.
‘putting together these insights will help police come up with a composite picture of the unsub’
Origin
1970s: abbreviation of unknown subject or unidentified subject.
So the crime reference is EARLIER than the opt out reference
Urban Dictionary:
“Our unsub is most likely a white male in his mid 30s, with a penchant for Star Wars action figures, and chocolate milk-type beverages.”
(Unknown Subject) The term used by Profilers in lieu of a suspect’s name.
Quora:
Yes, the FBI uses this term in real life, every single day. In fact, it is one of the bureau’s official terms used in FBI reports of investigations (FD-302’s).
Wikipedia:
“Unsub is an American television series that aired on NBC from February 3 to April 14, 1989. The series revolves around an elite FBI forensic team that investigates serial murderers and other violent crimes. Unsub is an abbreviation for the unknown subject of an investigation.
No, I had never heard of this show, which starred David Soul from Starsky & Hutch. It was on Friday nights at 10 p.m.
UNSUB: A Novel Hardcover – June 27, 2017
by Meg Gardiner (Author)
Caitlin Hendrix has been a Narcotics detective for six months when the killer at the heart of all her childhood nightmares reemerges: the Prophet. An UNSUB—what the FBI calls an unknown subject—the Prophet terrorized the Bay Area in the 1990s and nearly destroyed her father, the lead investigator on the case.
So, Dan, you are correct, sir. I had no idea.