No one is to engage in discussion or disputation with men of ill repute; and in the company of froward men everyone is to abstain from talk about (keep hidden) the meaning of the Law [Torah].
When these men have undergone, with blamelessness of conduct, a two year preparation in the fundamentals of the community, they shall be segregated as especially sacred among the formal members of the community. Any knowledge which the expositor of the law may possess but which may have to remain arcane to the ordinary layman, he shall not keep hidden from them; for in their case there need be no fear that it might induce apostasy.
dead sea scrolls, Manual of Discipline
These instructions in use at the time of Jesus clearly show that there was hidden teaching in the law, that was never written down, held back from their own laity and the rest of the world.
This is why Jesus said, "No one lights a candle and hides it under a bushel." A reference to the law being given as a light to the nations.
And that's why the religious authorities asked where Jesus got his teaching, not because it was unknown but because it was supposed to be kept secret, especially from froward people like Jesus. They couldn't believe that Jesus received this teaching from a Divine revelation from God himself.