Gremory managed to leave the library, not even Kluster saw him in the water sphere and he was also busy while he entered the secret passage on the library.
Kluster was a very busy man since he was a Grand Master, but lately, he was watching the entire Castle because of the political problems that he is having inside. He needed to stop the "Revolution" before it starts, a war inside the house would make them weak, even if he won.
In the end, all that matters was to be the stronger house. At least, few people knew about the water sphere, like Vendryt and some others. Giving the opportunity to watch his own enemy and see what they are planing.
By doing this, he knew who he could trust and who he would need to kill. Just as he did with Luther.
Luther was weak when using the house parameters, but still, he was a mage worth being trained. But Kluster weakened the position of his family by classifying him as not worth.