Ok, I found the reference. Its in ch 30 of FM-Denton and Harry are talking about how the belts have led the FBI agents to kill people and they sent Harry after the lycanthropes. Denton says 'After the deaths (the people the FBI agents killed the month before), I was warned about a governing body. A sort of magic police. The White Council. That you worked for them.'. So Denton was given the belts and killed people. Then someone warned him about the WC. Assuming he wasnt being helped by 2 unrelated people, whoever helped him was not hands off. Therefore, if it was Cowl, he shouldve known more then he did in DB.