Tap, tap tap. Scott Standfast knocked on the door again. He’d been at it for several minutes, standing in front of a one-story brick house in Niceville, Florida, on a warm, dry day in November 2015. The then-59-year-old former Marine and his wife had driven more than 11 hours to get here, hoping to answer a question that’s haunted him for 40 years. He knocked again. This time, harder. Boom, boom, boom! Still no answer.