Catching the early flight? Best stifle that yawn.

Blinking, coughing too often and even yawning too much could single you out for some difficult questions in security screening.

In the United States airport TSA agents are trained to look out for certain behaviours and nervous ticks that give away passengers as suspect.

Many commonplace behaviours such as just "throat clearing" are listed on the TSA's SPOT screening programme for passengers suspected of being security threats.

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The SPOT list – which stands for Screening of Passengers by Observation Techniques – is a 92-point checklist which agents use to identify passengers as potential security risks.

Among the other things agents are told to look out for include pale skin on passengers' faces "from recent shaving of beard" and "wearing improper attire" are other such markers for suspicion.

While some musical travellers may think they can carry a tune, whistling on approach to the screening area is a big red flag for the TSA.

The list which was leaked during a 2015 investigation by The Intercept revealed that many seemingly banal descriptions could cause you to be subjected to extra security screening at US airports.

Looking dazed or having rings under one's eyes might well describe any passenger returning on the late jet service home. However, they are all features that mark travellers out for extra screening measures.

Disorderly and disorganised travellers may pay the price of being extra suspect. "Arriving late for a flight" was highlighted as a key marker for the SPOT list, which could mean late-running travellers get caught up in security and miss their flight altogether.

However at the time of The Intercept report, the TSA said that they was no one behaviour that would trigger extra screening, but rather a combination of them.

"Looking for suspicious behaviour is a common sense approach used by law enforcement and security personnel across the country and the world, that focuses on those behavioural indicators, rather than items," they told the ABC.

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It is worth noting that both excessive blinking AND unblinking gaze are both cause for suspicion.

Looking for items that indicate stress, fear or intention to deceive, agents are told to award points on the 92 point SPOT suspicion score.

Carrying rope or cables can increase your suspicion score, as can "photo diagrams of high profile targets".

However, indicators of trustworthiness (or perceived harmlessness) can deduct points from the SPOT suspicion score.

Married couples travelling together are deducted 2 points on the scale. Female travellers over 55 and male over 65 are also indicated as lower risk travellers.

The one tell-tale sign that you're screening process will not be a smooth one is not to be found in your physical appearance, but printed on your boarding pass.

The letters SSSS are bad news. They stand for Secondary Security Screening Selection, which is triggered by factors such as paying for a ticket in cash or booking last-minute, one way.

TSA guide to potential terror threats

Exaggerated yawning

Excessive complaints at screening

Excessive coughing or throat clearing

Wide eyes and staring

Wearing improper attire

Whistling on approach to screening area

Gazing down

Exaggerated grooming gestures

Pale skin from recent beard shaving

Wringing hands together

No / little eye contact

Flushing face

Giving non-answers or being uncooperative

Trembling of voice or body

Questions or conversations that appear to be memorised

Pretending not to understand questions

Delayed response

Unfamiliarity with details of tickets or travel documents

Excessive perspiration