Sound exactly like leaf miners, to me. Do a comparison of some images and let us know if that's what you've been seeing.

That description does sound like the leaf miner, and the link below will take you to a picture of a leaf that has been worked on by one.
The leaf miner is the larva of a fly that lays an egg on the leaf surface and when that hatches the larva immediately burrows into the leaf where it is well protected from most anything except a wasp that can find those larva and then lays its own egg in that leaf miner. There is no spray that will control the larva, because they are in the leaf. The best defense against them is to make sure the plants you are growing are strong and healthy and can withstand this invasion.

Here is a link that might be useful: Leaf Miner

FYI: not always a fly; there are beetle and moth miners, as well.