During the course of the game, player has to activate them manually in order to bring them online. Following their activation, the M.A.D.D.'s assist the player in fighting off the Pfhor invaders aboard the UESC Marathon. Their weaponry appears to have effects similar to the player's assault rifle.
On the level Beware of Low-Flying Defense Drones..., Durandal says he has had an "accident" with some of the M.A.D.D.'s, and they turn on the player. These Drones are green-colored instead of the usual blue, have Grenade launchers instead of machine guns and they have significantly more health.
I have detected Defense Drones activating all over the ship. Reports from security forces are jubilant. Now, the Pfhor will encounter stiff resistance, and they will have to pay in blood for every move.
- Leela, from Marathon, Defend This!, Terminal 5 (success)
By the way, I had an accident with some defense drones while you were away. You might bump into a few of them here. Don't worry, they're mostly harmless; I don't think I gave them any ammunition for those grenade launchers.
- Durandal, from Marathon, Beware of Low-Flying Defense Drones..., Terminal 1