Speculatores and Exploratores were the scouts and reconnaissance

Reconnaissance

Reconnaissance is the military term for exploring beyond the area occupied by friendly forces to gain information about enemy forces or features of the environment....

 element of the Roman army

Roman army

The Roman army is the generic term for the terrestrial armed forces deployed by the kingdom of Rome , the Roman Republic , the Roman Empire and its successor, the Byzantine empire...

.
In both the legions

Roman legion

A Roman legion normally indicates the basic ancient Roman army unit recruited specifically from Roman citizens. The organization of legions varied greatly over time but they were typically composed of perhaps 5,000 soldiers, divided into maniples and later into "cohorts"...

 and in the praetorian

Praetorian Guard

The Praetorian Guard was a force of bodyguards used by Roman Emperors. The title was already used during the Roman Republic for the guards of Roman generals, at least since the rise to prominence of the Scipio family around 275 BC...

 camp, speculatores were initially scouts

Reconnaissance

Reconnaissance is the military term for exploring beyond the area occupied by friendly forces to gain information about enemy forces or features of the environment....

 but became bodyguard

Bodyguard

A bodyguard is a type of security operative or government agent who protects a person—usually a famous, wealthy, or politically important figure—from assault, kidnapping, assassination, stalking, loss of confidential information, terrorist attack or other threats.Most important public figures such...

s, courier

Courier

A courier is a person or a company who delivers messages, packages, and mail. Couriers are distinguished from ordinary mail services by features such as speed, security, tracking, signature, specialization and individualization of express services, and swift delivery times, which are optional for...

s, law-enforcers, and sometimes executioner

Executioner

A judicial executioner is a person who carries out a death sentence ordered by the state or other legal authority, which was known in feudal terminology as high justice.-Scope and job:...

s. Exploratores were tasked to keep watch on enemy movements in the field. Both occupations could require the wearing of 'plain clothes' and may therefore be deemed spies

SPY

SPY is a three-letter acronym that may refer to:* SPY , ticker symbol for Standard & Poor's Depositary Receipts* SPY , a satirical monthly, trademarked all-caps* SPY , airport code for San Pédro, Côte d'Ivoire...

 (Occulta speculator/speculatrix). To a certain extent (but only to an extent) speculatores may be deemed 'internal security' and exploratores 'external security.' However, the Roman Empire

Roman Empire

The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

 lasted a very long time and various espionage units, including the infamous frumentarii

Frumentarii

Frumentarii acted as the secret service of the Roman Empire. There are two main sources of information about the frumentarii, inscriptions on gravestones and anecdotes where the actions of individual frumentarii are mentioned by historians. From what is known of the Frumentarii, they always worked...

 created by Hadrian

Hadrian

Hadrian , was Roman Emperor from 117 to 138. He is best known for building Hadrian's Wall, which marked the northern limit of Roman Britain. In Rome, he re-built the Pantheon and constructed the Temple of Venus and Roma. In addition to being emperor, Hadrian was a humanist and was philhellene in...

to suppress internal dissent came and went throughout its history.