A Hole-y Question

Hey gang!
I have a plant with very small holes in two leaves. Two holes are on the third set, on the rightmost leaf. And on the newly emerging 5th set, the center leaf has two large holes in relation to it's smaller size. Now, before you go telling me I have an insect problem, I checked the holes with a 60x-100x microscope and I haven't found a trace. I've looked both under and on top of both affected leaves as well as doing a quick check up on the plant in general (with the microscope) and I can't find any munchers at all. I foliar feed and I'm thinking this could be signs of nute burn. Can anyone shed some light?

Alright, I know it's blurry but the hole can be seen where indicated. it's very small. Again, I'm aware of the lack of photo quality but the camera is a complete piece of garbage and [I]any[I] help would be appreciated.