As a reminder: Garland, Napier and Hart had coerced Emery into falsely confessing to Hannah’s murder (which was actually ordered by Napier) in order to protect the secret of Correction. Activist group the Pilgrims of Justice, along with a CIA whistle blower (who turned out really to be a CIA plant), had faked the footage of Shaun abducting Hannah to bring about his high profile arrest and subsequent release when they released the real video and exposed Correction. Frank Napier couldn’t afford for Correction to be exposed, so he had Hannah killed and Shaun framed for the murder.
Armed with the bus footage, Carey had blackmailed Napier, Garland and Hart into telling her the truth by threatening to send it to the Crown Prosecutor, thus invalidating the false confession Shaun Emery had made under duress after Napier kidnapped his young daughter. Garland and co. thwarted Carey’s blackmail attempt by using tech wizardry to corrupt the file he was positioned to upload at a pre-arranged time. “I’m sure you made copies,” Garland said to Carey, and she was right.
In series two, episode two, Commander Hart asked Garland if she trusted Carey. She didn’t, and cited “what we found” as a reason. Hart replied, “That was insurance, you’d do the same.” What Garland and Hart found, we learn at the end of episode three, was Carey’s hidden insurance, the memory card in the picture frame also containing a photograph of a young Rachel and her mother.
For context as to the photograph’s significance: Rachel’s mother died of lymphoma when Rachel was 13 years old, after which her father moved her in with the secret other family he’d had for years. Abigail, who comes to stay with Rachel in her bugged and secretly filmed apartment in season two, episode three, is her half-sister from that family. It’s little wonder that the dogged DCI can’t stand being lied to.