I had a scene where the investigators were interviewing a coworker of the deceased. The coworker said that he can recognize voices really well. It just flowed naturally out of their conversation. It wasn't anything I had planned. But I made a note to myself. "Chris recognizes voices."
And then two days ago I realized that was the clue to the last subplot. All the pieces are now clicking into place.
A while ago I read a book about writing (I still read a lot of those) and it said that when you got stuck, you should reread what you had written, because the solution was already there in characters and situations you had created.
I love it when it works like this.