There was no liquid inside the compartment when they pulled it out. Everything is bone dry. There is no odor whatsoever. If it was water or dirt, it would smell earthy. I took the car to another independent mechanic, he said it was the sulfuric acid that spewed out when the battery started leaking from the inside.
I'm guessing the yellow stuff is sulfur mixed a bit with rusted metal caused by the sulfuric acid? Since much is usually a darker brown that disintegrates with water?
I don't know. How can I test to see what that yellow stuff is for sure? Does the sulfuric acid inside car batteries look like that?