The other night I was out on a ride along with a LEO friend of mine when this came up. During a stop he asked the driver if he had anything in the vehicle that my friend needed to know about. I pointed out to him that if he had stopped me earlier that same day I would have answered no to that question. I told him that while I was carrying a moderate amount of black powder, under ten pounds, I would not consider that to be anything he needed to know about.
Now its not surprising to me that he agreed with my point of view. Its a legal product that I am transporting to another location. Therefore there is no need for him to know about it. It does not put him in any danger.
He told me that in his opinion he was trained to ask that question as a way to get people to voluntarily give up information that could incriminate them. When asked this question what exactly is required? I'm sure that what a LEO feels he needs to know is quite different from what I think he needs to know.