Taken from the game "Hitman," this exclamation was confined to the various Asian boards in the game and was said with a generically "Asian" accent. The full phrase was "weapon...weapon...watch out....maniac alarm!" The AI reserved such an exclamation for severe violations of civil decency on the part of the player/protagonist, such as walking into a Triad Den with a fully loaded ak47 and opening up for no good reason. The phrase can be applied, then, to any extremely antisocial behavior.
Everthing was going fine until you went totally maniac alarm on that dude.
Is that guy on the phone or is he yelling at no one? Ah, it's the latter. Maniac alarm.