Lack of motive?
Yeah, I was gonna say, well, they might not have actually been criminals.
But had the author met the wrong pair of dudes.........
I was thinking of an old Unsolved Mysteries late last night (actually early yesterday morning). Knew a guy who was related to the missing person in the story.
Looked it up - seems the guy has never been found. Went missing over 30 years ago. Wandered off. Perhaps he never wanted to be found again and simply went elsewhere on the planet. Dunno.