Spots on Tomato Seedlings?

Can anyone help identify what's happening to my tomato seedlings? These white/transparent spots have appeared on a few leaves. It's the same on top and bottom of the leaves, nothing brown or haloed atm, the spots look more clear then a true white as the photo deceives. The spots are much thinner then the "healthy leaf" areas. it is not "eatten" through as I'd expect from a pest. Plants are about 8 inches tall. Grown indoors on windowsill until last few days.   Help me save them?! Thanks ~ total noob gardener.