From Halopedia, the Halo wiki
Pre-Covenant models of Eyes used by the San'Shyuum were roughly diamond-shaped and could fly around freely. San'Shyuum commanders could observe battles from orbit by using the Eyes. Eyes were capable of projecting a holographic image of footage they captured while doing surveillance work. A San'Shyuum could interact with the hologram and highlight certain areas. They additionally kept information on certain individuals, if it was available. A San'Shyuum could have the Eye mark hostile individuals for assassination or deploy troops, orders that could be carried out by San'Shyuum forces or Aggressor Sentinels. Eyes could relay footage from other Eyes.
Various forms of Eyes continued to be used by the Covenant throughout the millennia, most recently in the Human-Covenant War. One such example used during the Algolis Invasion instead resembled a small sphere with antennae protruding from the sides, though was used for broadly similar purposes of reconnaissance.
The Eyes were used during the War of Beginnings. During the Skirmish of the Planet of Blue and Red in 860 BCE, Eyes were used to observe Sangheili forces. Eyes Seven and Thirteen were used to report on enemy movements during the battle. High Lord Mken 'Scre'ah'ben consulted with Eye Seven during the battle for an update on the Sangheili military's positions. While viewing footage from the Eye, Mken observed field commander Ussa 'Xellus. At first, the High Lord had the Eye mark the commander for assassination, but ultimately had the order belayed. Eye Seven was destroyed when Sangheili forces ambushed the San'Shyuum commander outpost. Eyes were later used by the Covenant to monitor Janjur Qom, the San'Shyuum homeworld. The Eyes revealed the Stoics' technological status to the rest of the Covenant. One Eye was once pursued by a Stoic aircraft on the planet.
During the Human-Covenant War, Eyes were used by Covenant forces invading the colony world of Algolis to spy on Atriox in combat with UNSC Marine forces. Following the war's end, they continued to be used by the Oath Warden Crei 'Ayomuu to track down the xenoarchaeologist Keely Iyuska inside the state of Vadam, on Sanghelios.
During the production of Halo: Combat Evolved, a model for an unmanned Covenant scout drone was created by the art team at Bungie, though ultimately cut from the game. Only a single screenshot and texture of the drone remain in the archives at 343 Industries, though to date these have not been released publicly. While broadly similar in purpose, it is unlikely that there is an explicit connection between this pre-production model and the Eyes first introduced in Halo: Broken Circle.