I have some leaf miners (A couple) and slight signs of thrips, etc but nothing critical. But I've never seen this before. All my other plants are fine by the way, it's only this one showing this particular symptom.
I might be wrong, but I don't think we have those here. I don't think the damage looks consistent with the pictures I find of Russet mite damage either. I haven't seen a single bug on the plant either. I appreciate the suggestion tho, I will check it again when the sun is out and see if there's anything I missed!