SCRANTON – Near the creek where Matthew Grendel’s body surfaced two years ago, sinister graffiti glared back at a retired detective on the trail of suspected serial killers. The 20-year-old college student’s disappearance sounded eerily similar to the other cases Kevin Gannon and his team investigated. The similarities brought them to Grendel’s house in Minooka and to Roaring Brook by the falls under the Harrison Avenue Bridge in 2007. The killers they were tracking left a crudely painted smiley face at the spot where some of the victims’ bodies were put in water. “We did find a smiley face down...