Does Subaru Wrx not have a TPM(Tire Pressure Monitor) installed? when I recently changed my tires the installer said it did not have any monitor. That made me think, how the low tire pressure indicator light on my dash board would work. The model I am referring to is 2009.
They use a rotation sensor, or rotation speed sensor. When the tire looses air it's circumference becomes smaller, so the sensor's signal changes and set of the light.
Added: See this site for full explanation Subaru's TPM