Sentinel launcher
From Halopedia, the Halo wiki
Sentinel Launchers are small armor-covered devices built into the walls of some Forerunner facilities. They have appeared in Halo 2 and Halo 3. When hostile entities, such as Flood, are detected in the immediate area, they open their cover and dispense Sentinels. When a Sentinel is about to be dispensed, the launcher will make a noise that increases in pitch until the Sentinel is dispensed.
Most Sentinels seen being dispensed are the normal silver ones, though a few dispense the gold, shielded Majors. One way to destroy a launcher is to attack it when the armored cover opens to release a Sentinel. It is also possible to destroy a closed dispenser by sticking a Plasma Grenade on its cover or meleeing it, although this will cause your shields to become depleted from the resulting explosion.
These launchers can only be found on the level Sacred Icon in Halo 2, and only briefly in Halo 3. This has confused a lot of Halo 2 players as on the levels The Arbiter and Oracle there are Sentinels but no Sentinel Launchers. Although some Sentinels could have escaped the destruction of the first Halo, few would have been able to. There are an extremely large number of Sentinels on the two levels, and too many for the amount that could have escaped Halo's destruction. This would probably mean that they came from a production facility of some sort or are produced on site, and are in fact Sentinels that orriginated from the Gas Mine.
