WikiLeaks has published a pair of internal CIA documents briefing undercover agents on how to dupe security at airports. The two documents — both classified as "Secret/NOFORN" meaning not to be shared with allied security agencies — give spies advice on how to maintain their cover. They also provide a detailed overview of the covert tactics airports use to vet travelers.

TOP CIA ADVICE: If you're a spy, try not to look nervous

Although some of the information in the documents is public knowledge, advice on how to avoid being singled out for secondary screening could be useful to a variety of people. These include tourists and travelers trying to get home for the holidays, but also terrorists, drug traffickers, and common criminals. The first of the documents — titled Surviving Secondary — covers everything from common sense advice about not looking shifty to warnings about more nuanced pitfalls that might turn a random baggage check into a full-blow investigation. These include:

Physiological signs of nervousness include shaking or trembling hands, rapid breathing for no apparent reason, cold sweats, pulsating carotid arteries, a flushed face, and avoidance of eye contact.

Lack of familiarity with passport entries (biographic page, previous travel).

Inability to speak the language of the passport-issuing country.

Purchase manner unusual to the place of issue.

Purchase or itinerary change within 24 hours of the scheduled flight.

Switching lines or studying security procedures.

And for luggage best practice:

An amount of baggage inappropriate for t he length of stay.

Multiple new items, such as alarm clocks o r notebooks, in baggage.

Carelessly packed baggage when passenger is purportedly an experienced business traveler.

Unopened and unmarked maps, g uidebooks, or other literature. Maps of unrelated cities in baggage for a purported t ourist traveler.

Camera quality not matching the traveler’s profile or camera memory card insufficient for a lengthy tourist trip.

Other tips are even more esoteric and focus on the specifics of certain countries' airport security. For example, Japanese airports pay close attention to lone Western travelers as they may be drug couriers, while in Iraq, any Kurdish passenger with a Turkish accent is "automatically sent to secondary" for fear they belong to a militant group called the PKK. In Mauritius, security forces watch passengers' facial expressions as they pick up their luggage, "zooming on individuals’ faces to study their expressions"; in Hungary, one-way mirrors are used to "monitor passengers for signs of nervousness," and in Bahrain, undercover officers sweep the arrivals lounge for uneasy travelers.

One agent was singled out for questioning because he was dressed too casually

If an agent is selected for secondary screening then they are simply advised to maintain their cover "no matter what." One anecdote details a CIA agent picked up for questioning because of "overly casual dress inconsistent with being a diplomatic-passport holder." Although security staff found traces of explosives in his luggage, the officer "gave the cover story that he had been in counterterrorism training" and was eventually let go.

A press statement from WikiLeaks comments: "This example begs the question: if the training that supposedly explained the explosives was only a cover story, what was a CIA officer really doing passing through an EU airport with traces of explosives on him, and why was he allowed to continue?"

The second document titled "Infiltrating Schengen" gives a brief history and overview of security protocols within the Schengen Area — a group of 26 European countries that have abolished passport control at shared borders. The report reveals that although most security measures target illegal immigrants, the CIA is worried about new biometric security measures set to come into force next year, including a fingerprint database that would make it harder for operatives to travel using multiple or false identities.