You are right with your assumptions but I don't know for sure despite dismantling every type of siren! The principle is that the siren is active all the time and is charging the backup batteries or capacitors as appropriate. Should power be cut to the siren or the module (either by a dead car battery or the wires being cut) it will activate and will continue to sound until it exhausts it's own power source. Logically as you have already realised this only requires three wires so assuming the Red and Brown are the live and earth respectively and they have a direct feed, that leaves the Violet/Grey, Black/White, Red/Brown and Brown/Violet to do the job of seemingly a single wire! What I haven't done yet is to measure the volatage of these wires at arm and disarm, this may hold a clue to their function.
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