A Retriever also known as a Strato-Sentinel was a Forerunner Sentinel variant. Retrievers were designed to strip mine the resource moon located in the middle of Installation 00 using an artificial gravity beam to remove the materials from its surface which are used to construct and repair various Forerunner installations.
- Retrievers were originally planned to appear in Halo 3, but were cut from the game.
- The word "strato" in the Sentinel's name is the Latin ablative form of "stratus," which means "stretched out" or "extended."
- One of the concepts for the Halo 3 multiplayer level Blackout had the map set in the partially-destroyed insides of a Retriever orbiting the Ark's strip mining moon.