if it turns on when suddenly speeding up or slowing down, it means the rear brake pads are almost dead. the sensor is hitting the backing plate and sending a signal to the ecu that the ebrake is being used. it's REALLY not, but it's just a warning that the pads are due to be changed.
Yeah, definitely the rear pads. I let mine go for way too long on my 04 Forester and the same thing happened. Get them replaced before you have to replace your rotors, yo.