If you're in Starbucks and notice a nondescript individual paying for their coffee with cash - watch out.
According to the FBI you might have found evidence of a terrorist plot.
A series of fliers distributed to companies around the United States by the FBI and the Bureau of Justice Assistance appear to give workers and business owners exactly this advice.
The fliers, as highlighted by the miscellania blog BoingBoing, are intended to help various businesses from hobby shop owners to car rental services identify suspicious people who might be involved in terrorist activity.
They have been collected online by the Public Intelligence collaborative research project, and are not normally released en masse by the FBI.
The fliers include much salient advice, such as pointing out the need for valid ID from customers purchasing large quantities of chemicals.
Reassuringly each of them also states that "it is important to remember that just because someone’s speech, actions, beliefs, appearance, or way of life is different, it does not mean that he or she is suspicious."
However the fliers also reveal the slightly more bizarre side of counter-terrorism.
For one, the fliers advise internet cafe owners that "evidence of a residential based internet provider (signs on to Comcast, AOL)" could be a sign of a suspicious character. The FBI says that such services can mask IP addresses, and that may be a sign the person does not want their tracks followed.
The Huffington Post UK is owned by AOL - but thankfully we didn't make it onto the list.
People who pay cash for small purchases, including coffee, are also highlighted in the fliers. They may be attempting to avoid a paper trail of their whereabouts by not using credit cards.
Owners of electronics stores visited by regulars with missing fingers, chemical burns or strange odours are also told to watch out.
And if you're a paintball proprietor and you have a regular customer who alters his appearance ("beard shaved off, hair colour changed") on each visit, you might also have a dangerous character in your midst says the FBI.
Tattoo parlour owners are asked to watch out for groups requesting the same tattoos or placement of tattoos.
More obviously, perhaps, the FBI warns paintball range owners that if groups want to mimic security checkpoints, practice kidnapping or refer to jihadi training materials during games, that might also be a clue something's up. Which seems fair.
Proprietors of hobby shops are informed that anyone purchasing large quantities of model rocket motor igniters or model aircraft fuel might be suspicious.
The full range of fliers so far gathered together can be read at Public Intelligence.