I had this same problem last season. It's beginning early this time. In the photo, you can see the holes in the leaves of my turnip seedlings. By harvest time, they'll look like fine lace. I can't find any little worms or eggs. I wouldn't mind, if I only wanted the roots, but I like the greens. What would likely be doing this? I don't think it is leaf miners or aphids, but I could certainly be wrong.