last sighting? a colleague saw him driving his black mercedes. >> we felt that the vehicle was still out there somewhere. >> reporter: so find the car, and maybe find out what happened to gavin. and nine months after gavin vanished, they finally got the break they were looking for. >> i said, "i'm here to talk to you about your daughter." and he started crying. and he said, she saw everything. >> reporter: coming up. >> we waited until she showed up and kind of surprised her. very nervous. startled. >> a new witness and a game changer of a clue. >> i kissed my partner on the cheek. it was pretty exciting. big piece of evidence. huge moment. >> you think you can't be shocked. but you can be. >> when "dark valley" continues. .