It doesn't look like anything too serious to me. Definitely not fungal, but might be leafminers. They feed on the mesophyll layer of the leaf, between the top and bottom layers, and often get caught in between leaf veins like you're seeing on the farthest left leaf. Not much you can do about them I believe, and they're not terribly destructive.
I'm not 100% about this though. Just keep an eye on them, and if it seems to be a problem that spreads, get to googlin I'd say. The leaves in question otherwise look healthy and nice and green, so you're obviously doing some things very right!