Looks like thrip damage.
I've had them loads and know it quite well unfortunately.
They are incredibly tiny at the start of their lifecycle and difficult to spot, but find they usually hide in the veins of the leaf (on top) or in the groove down the petiole is their favorite highway to the leaf. Spider mites, aphids and whiteflies love hiding under, but thrips seem to favor the top.
Poison sprays are not very effective with them, predatory mites are best, otherwise a foliar spray with Diatomaceous Earth.
Luckily they are not a massive problem as long as you don't let them get out of control, but not having them is even better. :P