welcome to HCM
Thanks for the concern.
Its nice to read that you are leading a quite healthy life style.
sir we usually see these kind of patches in associated with long term tobacco use, can be chewable/smoking
.as you are a non smoker dont worry. it can be melanin
pigmentation or some kind of metallic reacction patch, which can be diagnosed only after seeing it clinically.
so its better to consult your dentist
for the opinion and treatment.