Won’t lie, would kick in my OCD. But, KC is a meh product, it’s more of a “topper” than a cleaner. Try using something like KC Green Star or Pol Star as an interior cleaner. Can get away using it 10:1 with no problems, though some will use it 15:1-20:1. If you want something that’s already ready to use, Armor Detail Piet is good, along with Gyeon Purify Interior (odorless). Have a new one to try coming in, so can’t give any thoughts on that one.
that looks like something (natural oils ???) on your fingers doing discoloration over a long period.
since the volume knob is the exact some color discoloration and a small area that fits the size of a finger, that is what i am leaning to
i personally have an issue where my face produces excessive oils during the day. if i dont clean my face on a daily basis or evey twice a day, i can easily get a stye in my eyelids. had it happen before and i had to have one lanced as a pore got blocked and hardened. doctor told me to wash minimum once a day or ideally twice.
doesnt help i wear glasses either. so i get oils on my fingers from my face which in turn touch things.