sandi johnson's family and friends would agree. the young mom was petite, pretty, and devoted to her two kids. but when she went missing, police wondered if sandi's home life was as happy as it seemed. they quickly uncovered a web of romantic relationships and a dark mystery that would take years to solve. here's andrea canning with "the disappearance of sandi johnson." >> it's a celebration no mother would ever want to miss. a child's 5th birthday. but suddenly, she was gone. out of his life. >> there's nothing that anybody could really do to fix that or repair that missing part. >> his mother, sandi johnson, was outgoing, energetic, and an awesome mom. >> those kids were everything to her. >> so, why wasn't she there to sing happy birthday to her son. >> all they knew was that she just disappeared into thin air. >> she should have seen you get married. she should have seen you go into the military and become a police officer one day. >> it's the toughest thing i've ever had to deal with. >> it was heartbreaking and baffling. >> it