Pelant told Brennan that she would find him in the stars or something like that... She put the pieces together from the remains that Pelant told her about, and Aquilla (?) which was the bird that ate the liver of Prometheus, and is also the name of a constellation. The victims were each from different cites, and when lined up with the constellation, the stars equated to each of those cities, with the main and brightest star being the location of the power plant. Like Cam said, Pelant left Brennan a love letter that only she would understand.