It looks like something has been chewing on the leaf. Do you have snails or slugs? Caterpillars also do this type damage. How about wasps called yellow jackets? They chew leaves and build nests out of the material.
Unfortunatly I have all of the above and then some in my yard. So keeping the greenhouse safe from them is hard. I had spider mites for a wile but those are all gone now. The only thing that throws me off is that it looks lije the leaf is just sunken in. Not bitten into
Could it be cell collapse from cold water? I would just keep an eye on it. Doesn't look like any bacterial or fungal problem. I haven't seen virus do this type damage either.