The purpose of the 10 character minimum is to encourage you to post something of significance.
You don’t have to reply to each individual, we all see your answers.
@cuzzzzz is responding via email, I don’t think he can respond to multiple posts in a single post.
I would say it was just rust buildup on the rotors or the brake shield on the backside of the rotors that just rubbed off after you drove it for a short distance .
A glance at the car’s registration will reveal both the model year and the model name of this car.