I just
had to take a look, haha. I ended up redrawing the entire circuit to make better sense of it
I wanted to say for sure that it never shuts off, but I found one wire I can't figure out and it ties right into the alarm sound section. It's node "N" on the schematic. It ties to the input of what would be the 11V regulator during normal use, and would seem to want to light up the power LED, which we know doesn't happen in alarm mode. Either the power/sensor LEDs are labeled backwards, the switches are not following their on/off legend at the bottom, or there's a wiring error.
I haven't figured out yet which mode each op amp section is set up for to understand their individual function, but I think the first two act as the timer and the third one acts as the trip switch, and the fourth possibly to support the siren circuit.
Once you press the alarm button, the power supply in the lower-right is enabled. This allows the timer circuit to start. Something slowly builds to a point where a threshold can be crossed by tripping the sensor, which I suspect is happening at Q206. Once that transistor turns on, it enables a signal through the op amp stage it's attached to, which simultaneously enables power to the amp chips by way of Q211 and Q212, and the siren audio circuit by way of the last op amp stage. (
full schematic here)
