Where is the proof that a man can marry a second wife without the first knowing?

I have seen some Islamic lectures that say that the man does not need to ask his first wife's permission, but also the first wife does not even need to know? Isn't this lying and deceit? How does such a thing become valid when it comes to a man? I understand the proof that he does not need the permission of a wife is the fact that the Qu'ran makes it his right to marry more than one. But is it valid outside of the knowledge of the first wife? Also does it count as unfair if the second wife knows of the first wife, and the first does not?