you need to replace the sensor because it was not replaced during the last pad change.
The sensor is a 1 time use sensor. It wears down against the rotor when your pads are low enough and trips the light on the OBC. Get a new sensor from the dealership or bavauto or some place and replace the worn out sensor.
You have two. One in the front driver side, one in the rear passenger side. Only one needs to be worn out for the light to illuminate.