A cue from an old cartoon show that was a parody of the spy genre; one of the characters was a
squirrel spy. Originally meant to refer to someone working in covert operations or intelligence (undercover
police, CIA, military intelligence, etc).
It is now often used to refer to
people who claim to have worked in the intelligence community in online forums and in person out of context with questionable veracity - ie potential/real poseurs/frauds.
Forum poster X - " I worked in intelligence for 5 years and served in
special operations but my service
history is completely classified."
Forum poster Y -" Oh great, another secret
squirrel."