Heat spots IMO.
"Heat spots (Fig. x) are a tell tale sign that your brake rotors have uneven brake pad deposits. How do they affect you? Heat spots will lead to brake harshness, vibration and reduced structural integrity. Heat spots on brake rotors occur when brakes aren't broken in (bedded) properly and the brake pad deposits on the rotor accumulate and eventually turn into a compound known as cementite. You don't want Cementite on your brake rotors. Cementite is compound that is very hard, overly abrasive and doesn't allow the rotors to cool properly. As a result, Cementite heats up the local area around itself growing in size and reducing the cooling capacity of your brake rotors.
Prevention: Properly breaking/bedding in your new brake rotors is vital to the longevity of the rotor. See our Brake Rotor Bedding Guide.
If this is caught early on, you can possibly remedy it by bedding in your brakes with a more abrasive pad, or resurfacing your rotors. If the heat spots are extensive, you will need to replace your rotors."
https://brakeperformance.com/inspect...brake%20rotors.