That's what we need to figure out. When I have time I'm going to open the panel and with a multimeter try to determine how the sensors are wired and operate. It may be 3 conductor [ground, DC power, signal (ON/OFF)] or simpler.
I work on fire alarm systems, security systems, and other life safety equipment.
Most modern security panels have 8 zones on the main board... usually operating at 12-16vdc. A typical zone is 2 wires... a common and a return. Depending on the configuration of the point in software, the point may be normally-open unsupervised OR supervised, OR normally-closed unsupervised OR supervised... Supervision is done with a resistor and offers 3 states NORMAL, OPEN, and SHORT. A non-supervised only offers 2 states NORMAL and OFF-NORMAL.
Most panels work about the same way when it comes to the zones, with minor differences in the End-Of-Line (EOL) supervision resistor value.
now wasnt that geeky!