So the author still insistent with the "second traitor". I thought he would give it up when Yutori reveal her trueself. The possibilities that we are left with:
1. Shiba. We are now being led to believe that the traitor is him. He probably left to look for Tenji to trap/misled him before he meets Shiho.
2. Tenji. For whatever reason, as much as I hate this possibility, it have bigger chance of happening. It will create even more twist and author sure do love some twists.