"Who did you think Spidey was? ;)"
I know you didn't ask me but I'll answer anyway.
Given that Justin's friends knew about the notebook, I didn't see any of them leaving it there if they had found it. Especially if they had read it.
That left only Eyal, Theo or someone we hadn't met.
(And I had hoped for the third option, that Spidey actually had nothing to do with the Hunt and he would be another dimension of the situation that Justin would have to deal with.)