Rex left the cafe after he talks to the Reed Family, it surprisingly went well.
He already thought of how to attract the Reed Family's attention to him but who would've thought that the detective came to him himself, it makes his plan to attract the Reed Family becomes unnecessary.
From the sudden interest of the detective, Rex already has someone in mind.
'It must be the principal, he didn't even hesitate to tell the Reed Family about my possession of the spellbooks', Rex thought.
The principal knows that he has the spellbooks from the exchange before, so it makes sense for Rex to put the principal as the main suspect.
But that doesn't matter anymore.
The thing that matters is that his plan becomes faster thanks to that, he manages to make a deal with the Reed Family because of it.
He then went to Emham Forest without worry now.