Sentry bots were programmed to enforce perimeter security under all circumstances. A single sentry bot will cut down an ISA platoon in seconds. They use two mounted light machine guns where arms would typically be and fly all over places to kill you. If they were to touch anything they will explode, a few lucky shots from a shotgun or heavy weapon should see the sentry bot to its destruction.
They appear in Killzone 2 and 3. In multiplayer, they are replaced by Tactician's Drones.