A letter dated to the eighteenth century BC sent by a servant to Zimri-Lim, the king of Mari, details a system of long-distance signal fires, thought to be the first ever in use. 

Walter Kirke, British deputy head of military intelligence in France, noted in his diary in October 1915 that the chief (“C”) of the Secret Intelligence Service had come upon a solution for how to send secret messages: “Heard from C that the best invisible ink is semen,” Kirke wrote. The substance, it turned out, was hard to detect by the common revealing method of iodine vapor. The chief’s name: Mansfield Cumming.

Concerned about pigeons carrying military communications, German troops in occupied Belgium during World War I would shoot at overhead flocks. Such fears had not abated by World War II, when the British government ordered a systematic slaughter of pigeons throughout the UK, and inmates at British and Australian interment camps were banned from approaching birds on compound grounds. 

Seventh-century Persian king Khosrow II is said to have tested the loyalty of courtiers whom he believed were becoming too close. Telling one of his decision to execute the other, he would swear the man to secrecy and then watch the friend’s behavior. If it went unchanged, he knew the first man was loyal and had kept silent; if different, he was a traitor and dealt with accordingly. 

Union general William T. Sherman believed newspaper correspondents to be liabilities. “A spy is one who furnishes an enemy with knowledge useful to him and dangerous to us,” Sherman wrote in an 1863 letter. “I say—in giving intelligence to the enemy, in sowing discord and discontent in an army—these men fulfill all the conditions of spies.”

To send a confidential note to Cyrus the Great along heavily guarded roads, sixth-century-bc Median noble Harpagus inserted a paper message into a dead hare’s belly and ordered a servant to pose as a hunter to deliver the corpse.

In 480 bc, with the Persian army on the cusp of defeating Greece, Athenian general Themistocles sent a trusted slave to convey a message to Persian king Xerxes; the note professed allegiance to Persia and reported many Greek ships prepared to defect. The Persians, acting hastily on this false intelligence, sailed into the Strait of Salamis, where the Greek fleet was waiting and gained a decisive victory. 

While stationed as a secret agent in Bern in 1917, future CIA director Allen W. Dulles received a phone call from a Russian exile with an urgent message to deliver to the United States. Having already arranged a rendezvous with Swiss twin sisters at a country inn, Dulles demurred, finding out only later that the caller was Vladimir Lenin, who returned to Russia in a sealed train the next day. 

“Secretary Morrice did this day in the House, when they talked of intelligence, say that he was allowed but £700 a year,” wrote Samuel Pepys in his diary in 1668, “whereas, in Cromwell’s time, he did allow £70,000 a year for it; and was confirmed therein by Colonel Birch, who said that thereby Cromwell carried the secrets of all the princes of Europe at his girdle.”

In ancient Indian espionage networks, a sattri was an orphan-spy trained from youth in palmistry, magic, omens, and augury. 

At Super Bowl XXXV in Tampa police used cameras and biometric face-recognition software to survey fans. The system identified nineteen subjects of outstanding warrants, far more than anticipated. None were arrested. “We thought we were ready to use it,” said a detective.

A sect of violent Islamic agents founded in eleventh-century Persia to fight the Fatimid caliph in Egypt became known as the Hashshashin, which translates as “eaters of hashish”; their grand master supposedly administered the drug to demonstrate the paradise that would follow agents’ service. From the name Hashshashin, Christian Crusaders derived the word assassin.

When asked why he didn’t use intelligence agents, Alp Arslan, sultan of the Seljuq Empire in the 1060s, replied that his favored subjects would trust the spies, while his opponents would curry favor and bribe them; he’d end up hearing damaging reports about his friends and positive ones about his enemies. “Reports good and bad are like arrows,” Arslan said. “If you shoot enough of them, at least one will hit the target.”

About 40 percent of employers peer at the social-media profiles of prospective candidates, and 40 percent of party guests admit to snooping through a host’s medicine cabinet or drawers. The percent of women who admit to having looked through a partner’s phone without permission is 34 percent; of men, 62.

Michael Hayden, former director of the CIA and NSA, claimed while discussing the NSA’s collection of telephone-call metadata, “We kill people based on metadata,” quickly qualifying, “But that’s not what we do with this metadata.” When declining an interview about alleged U.S. cyberattacks on Iran, he sent a one-line email that read, “Don’t know what I would have to say beyond what I read in the papers.”