Are you perhaps thinking of the chevauchée? The term chevauchée generally refers to the kind of long-range raiding activity you describe, although the force in question doesn't necessarily have to be large in size or encompass a huge span of territory. Frequently, smaller detachments would break off from a larger force to extend the reach of an army engaged in a chevauchée. Raiding warfare of a similar sort is found across the entire medieval period and is not exclusive to the Hundred Years War. In the medieval Iberian peninsula especially, raids and counter-raids (often by relatively light footsoldiers or cavalrymen) accounted for a large percentage of combat actions. The Spanish called their version of this raiding warfare a cavalgada, while Muslims refer to it as a razzia. English strategy in performing chevauchée was somewhat unusual. While many commanders might utilize raids as a way of weakening the enemy at a low risk to themselves, the English often performed larger-scale chevauchées for the purpose of forcing the French into a pitched battle in the open field (which they generally felt confident about their ability to win). The campaigns preceding the famous battles of both Crecy and Agincourt are the best examples of this strategy.