It would follow that both the Mage of Exodus was involved as well as someone being present from the Kusaribe clan. By the rest of the Kusaribes not detecting anything, they could conclude that something was interfering, that is, the Mage of Exodus. That Hakase detected a Kusaribe clan member is that much more significant, because she is magnitudes stronger than the rest of them are. Someone from the Kusaribe was present, but not directly involved.

In fact, this might support Arch's postulation that Aika was the Mage of Exodus before her death by suicide.

Either that, or Hanemura Megumu is lying about both his disposition for violence and destruction and everything else we've seen. I severely doubt he is that phenomenal an actor. He appears to be a genuinely nice guy who realized he had the power around the same time Aika was killed.