I do not know all there is to know about this, but when you have an FFL at your home it has to be open at times and the ATF has the right to come inspect and audit your books. I think probably more to the story.If I remember the story right, this particular FFL consented to them coming to visit him. Thus, I don't think a warrant is necessary in such cases.
But, I'm not an FFL, nor plan to be one, so what do I know about it.