We are finding car radiators with corrosion pin holes in the cores. In some cases we find stray currents in the cooling system and can attribute the problem to electrolysis. In other cases we get an absolute zero reading, yet when we cut the tubes open we can see clear signs of corrosion pitting. We have read that depleted anti-freeze can do this, but the glycol in anti-freeze is not corrosive. By what mechanism can this form of corrosion occur in a radiator?

Most times I've found electolysis due to current so small that I could not measure it.
True, the glycol in anti-freeze isn't corrosive but the water component still is.
Another concern that often goes unnoticed is simply that the metal is manufactured too thin and will ablate until a pin-hole opens. I've seen this before and I'm certain I'll see it again. Skimping during manufacturing will save money initailly, but it will cost you customer loyalty in the long run.