I encountered the problem of very small surface defect around 40 years ago.
It was a kind of scab of which size was, maybe from my remembrance, 5-10mm*20-40mm and very shallow. The scabs existed all over the pickled strip surface, only one surface or both surfaces, I do not remember exactly now. After all kinds of survey and experiments at the hot strip mill we could not find any reason of the defects. Also we could not find any small cracks on the surface of slabs.
But, study using EPMA only revealed Fe and O, no sign of inclusion.

If the defect was generated by the operation at the hot strip mill, there should have been some tendency in the distribution of the defects due to temperature distribution, such as more occurrence ratio at head end, latter half, both sides of strip in width direction and so on. So we believed the defect was generated at steel making division, but we could not persuade them.

Finally we did new approach. At that time the hot strip mill had the equipment to pickle a slab for making special product. We used the equipment. After pickling the slab, we found very small cracks spattering all over the slab surface. They looked like very small spiders.
The evidence helped steel making division recognize the origin of the small cracks had been made by the direct contact of mold cupper and liquid steel. It was found that Ni plating of the cupper mold had been worn out. We experienced directly copper embrittlement phenomenon for the first time.