Just thinking. Brian comes home without Gabby, his parents drag the truth out of him. His dad gets pissed at him, they get into a brawl. Dad unintentionally kills Brian. Dad and mom dump Brian's body in the swamp, along with his backpack and notebook and tell law enforcement that they think he is hiding in the swamp. They wait long enough for the gators, crabs, and bugs to chew up the body to obscure the cause of death. Then they "helpfully" assist LE in the search and "find" the body parts and personal items.
Case closed.
