I have a Civic with a little over 20K miles with the original brake pads and rotors. One of the rear rotors has "step-like" grooves on the top and bottom. I visually inspected but can't confirm if there is any brake pad material. The strange part is that all of the other rotors have even wear and are shiny with good brake pads. When driving/stopping, I do not hear any unusual sounds like grinding and squealing and there is no vibration. I went through some of my older pictures and it looks like similar marks were on the rotor a year ago (though, unclear if they were less severe).
I confirmed that the caliper is operational and stops the rotor from spinning when the brake pedal is pressed.
I am including pictures for reference. What could have caused this type of damage?