Ignorance of the law is not 'mens rea'. Those who have a sub-normal mental ability can steal without understanding that this is wrong. It is the intent to do wrong that is the mental awareness. Much wrong is also against the law, but some isn't, and yet most people can draw a line where right shades into a very narrow band of questionable before we're in the territory or wrong. Cleaners who mistook the house clearly had no idea of doing wrong. Those who claim they didn't know it was wrong to steal a co-worker's packed luck clearly have a mens rea that they crossed the line, because nobody who knows the 'rules' of the workplace (and decent behaviour) can pretend they didn't know stealing was wrong.