Thanks for the info. Yeah, I wasn't sure if it was an explosion of battery acid or what. Then I thought windshield wiper fluid but wouldn't think that would cause that kind of damage.Carmax will probably say their price factors in the condition under the hood. True or not, won't matter; they're fine letting you walk. Good idea for a thorough inspection, especially for wires, hoses, insulation, etc. I would walk if they find anything.
I just looked under my hood. The dirty spot on the hood liner is above a cavity of the battery and core support. I don't think it's related to the battery. Maybe some dirt/pollen that found it's way in the engine bay.