The Sentry Bot is a security robot used in all places of Mars City in Doom 3. They are equipped with a Machine gun and a Flashlight. They were constructed by Mixom Corporation to supply the UAC. However, there were many problems with them. For example, a UAC employee was almost killed by a Sentry Bot, because it had a system failure. Fortunately, a nearby marine was able to disable it.
At the beginning of the game they can be seen in the marine HQ. One Sentry Bot will follow the player to the Mars City Underground elevator. When the demons invade the base, most of the Sentry Bots are destroyed. If you find one, it will help you fight the demons.