Brakes Squeak inconsistently

2017 Toyota Camry. Also no its not happening after it rains. So my break pads and rotors were changed maybe 2 months ago. I have an inconsistent squeak that can be low sometimes and other times sounds like a braking school bus and other times theres no noise at all. It took maybe 3 or 4 weeks after changing before the noise started. These pictures of my back rotors show that on my left side, theres a line developing. Seems not good to me, any suggestions on what the issue is or what i should check? I included pics of my old brake pads because the way the rear pads were worn down seemed odd to me but i was told not to worry at the time.. but def seems like it could be a consideration now