Tell No One

Alexandre Beck is a doctor who has slowly been putting his life back together after his wife Margot was murdered by a serial killer. Eight years on, Alex is doing well, until he finds himself implicated in a double homicide, which has plenty of evidence pointing to him as the killerÂ â though he knows nothing of the crimes. The same day, Alex receives an email that appears to be from Margot, which includes a link to a surveillance video clip that features his late wife looking alive and well. The message warns Alex that they are both being watched. He struggles to stay one step ahead of the law, while henchmen intimidate Alex's friends into telling them whatever they might know about himÂ â the henchmen eventually kill one of them, Charlotte. In the meantime, Alex's sister Anne persuades her well-off wife HÃ©lÃ¨ne to hire a respected attorney, Ãlisabeth Feldman, to handle Alex's case.
It is gradually revealed that Margot is apparently still alive. She attempts to arrange a meeting with Alex by sending him an email which he must read in an internet cafe to avoid being spied on. Before this meeting, a warrant is issued for Alex's arrest for the murder of Charlotte. He goes on the run whilst his friends and lawyers struggle to find out the truth about the murder, as well as Margot's reappearance. Alex, chased by police officers, is rescued by Bruno, a gangster from a rough part of the city who feels he owes Alex a favor. The mysterious henchmen reappear to prevent Alex's meeting with his wife, but he is rescued once again by Bruno. Margot is seen almost escaping on a flight to Buenos Aires. Elizabeth, the lawyer, proves that Alex has an alibi for the murder of Charlotte, thanks to eyewitness accounts at the internet cafe.
Alex noted the numerous mysteries about his wife's deathÂ â mysterious photos of her covered in bruises and traces of heroin in her body. He soon discovers the truth that Margot's father faked his daughter's death. Margot had discovered that Philippe Neuville, the rich young son of a local aristocrat, was a pedophile rapist whose activities were being hidden because his father had influence over the police; when she confronted him, Philippe beat her up, causing the bruises. Her father explains that he walked in on the beating and shot Philippe. The elder Neuville hired thugs to kill Margot. Margot's father knew this because he tapped the phone call, so he doubled the payout for one of the thugs to fake Margot's murder instead, kill the other thug, and knock out Alex in the process. Margot's father then shot the second thug and buried both, then used the body of a dead heroin addict to stand in for Margot's.
Police, listening in on the father's confession, attempt to arrest him. Margot's father shoots himself dead before he can be arrested.
It is revealed that Margot's father knew Alex was wearing a wire, and that during a moment in which he had blocked the bug's transmission he had told Alex one last thing: it was in fact Margot who shot Philippe after he beat her; her father was covering up her crime, not his. His actions have ensured that she will never be suspected. Finally, Philippe's father is arrested, and Alex and Margot reunite at the lake where they fell in love as children.

Question: Traces of what were found on Alex's wife's body?
Answer: Heroin