This is a story related achievement which will unlock with natural story progression.
After you taken control of Watson for the third time after the second murder, you will need to go to the police station once again. Humperies will tell you that he has torn out a several reports as they were no longer required. You will also see a witness sitting beside him. After you have dealt with Bella and visited the Wasp Nest Pub you will be able to return to the police station to collect these reports.
You will need to piece the reports back together. This is a fairly straightforward jigsaw puzzle which requires little decrypting.