Does the 2009 Subaru WRX 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 wonder, how does the low tire pressure indicator light on my dashboard work?
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 sets off the light.
Added: See this site for a full explanation of Subaru's TPM