"When it came to gathering intelligence for the Roman army, the exploratores and the speculatores were the units...

"When it came to gathering intelligence for the Roman army, the exploratores and the speculatores were the units most often used. Although each unit had specific duties, their jobs did tend to overlap from time to time.* Exploratores* were generally used to gain information through the use of patrols at a distance from the enemy, whereas speculatores most often acquired enemy intelligence through undercover operations (Evov, 81)."
https://legioilynx.wordpress.com/2012/02/28/roman-scouts/