The first chance that you get at unlocking this achievement is during "The Driver's Seat" case at the Traffic Desk.
After investigating and finding all the clues at the main crime scene at the beginning of the case, you are given the objective of traveling to Adrian Black's residence. Once you arrive, knock on the front door and you will enter a cutscene. Once you finish the scene, enter the house and walk straight forward into a messy bedroom. On a dresser in the left of the room, there is a picture of Mr. Black. Pick it up and examine it further by pressing
, which will prompt you to open up the picture and find a secret love letter in the back.
After investigating the rest of the house, question Mrs. Black in the living room. When you get to the question regarding the clue "Photograph signed Nicole", Mrs. Black will lie and you must choose the option of Bad Cop (
) and present her with the evidence of the "Concealed Message". Achievement unlocked.