It depended a lot upon the circumstances. If you read Caesar’s Gallic Wars you see how Caesar often depended upon merchants for intelligence. These were active traders beyond Rome’s borders and knew the terrain, the people, and what they were doing.

Probably the most famous domestic agents were the frumentarii. These were military agents who acted as wheat collectors (frumentum – supply of grain) but eventually took on intelligence duties probably around the time of Trajan or Hadrian. The Romans were practical people, and who would hear more rumours than agents who travelled around their provinces talking to the citizens?

Novelists have tried to portray the frumentarii as a type of Roman FBI or KGB, but this has probably been exaggerated. They proudly put their status on their tombstones, which would seem odd if they were ‘secret agents’ as we think of them today. They also seemed to always work in uniform or military insignia. Imagine if all FBI agents had to wear a uniform, and you get the idea that a secret agent role would be pretty difficult. And, of course, they still needed to collect grain.

But we do know they were involved in intelligence gathering at times. They are mentioned several times in the Historia Augusta in such a role. One example was when Hadrian found out a woman had wrote to her husband, complaining how he spent all his time in the baths and his own pleasures instead of spending time with her. The Emperor rebuked the man, who was amazed and asked if his wife had written the exact same thing to Hadrian as she did to him. (HA, Hadrian, 11.4+) The HA should be treated with caution as a source, but I think it is generally accepted that the frumentarii sometimes acted as domestic spies.

At times they also had other odd roles. There is an inscription from Ephesus about one who was in charge of a prison. Other inscriptions show a few who were in charge of military building projects.

People do not generally like people who spy on them, and according to Dio the frumentarii seemed to have acquired an unsavoury reputation by the third century.

There are some questions on the organisation of the frumentarii, with some scholars thinking they were solely under the command of the emperor and others thinking they were under local governors. They had a headquarters, or at least a base, in Rome.

Sources: Caesar Gallic War, HA; Dio; Rankov, Frumentarii, the Castra Peregrina and the Provincial Officia; J.C. Mann, The Organization of the Frumentarii