[Mouri only realized there were two victims when Lynne was the only other person that wasn'tin the room. He takes a moment to mourn the loss of the other detective, but there was not much time for that. It was trial time]

I investigated the scene of Tarrlok's murder and...the conclusion I got to is that the body was moved. It was laid too neatly for it to have fallen down the stairs. It is possible he was killed somewhere else.