Canary trap

Seeding each copy of a document with unique markers to identify a leaker

Canary trap is a method of exposing the source of a leak by giving each recipient of a sensitive document a slightly different version, so that whichever variant later surfaces identifies who passed it on. The older, broader espionage term for the same idea is the barium meal test. (The unrelated tradecraft phrase "passage material" refers instead to information fed through a double agent to the opposing service, not to leaker detection.)

Deep-program insiders such as Bob Lazar have said the classified briefing documents they were shown may have been individually peppered with unique, unverifiable details — a canary trap — so that any dissemination could be traced to the specific reader. That possibility is offered, in-universe, as a reason such insiders treat the contents of their own briefings cautiously, and as a mechanism by which a sanctioned leak can be both permitted and, if it strays, traced back to its carrier.