If You See Me by Audra McElyea

When Leighton Marx is taken at a festival in front of her loved ones, police are quick to label it a random trafficking incident, but Leighton's husband, Owen, and her ex-best-friend, Marnie, suspect there's more to the story. After Marnie meets Sarah, who saw something at the festival that doesn't add up, she starts looking for Leighton herself-only to uncover secrets and inconsistencies surrounding her disappearance that grow deeper and more complicated by the minute. As Marnie continues to search, she receives threats from two parties with conflicting agendas, and she must decide whether she should risk it all to find her friend or if she thinks it'd be better for everyone if Leighton stays gone after she learns the whole twisted truth.