Once back in Hobbs' studio, go up to the balcony door and use the chain on it. George will bind the chain to the door and elevator and send the elevator down to force the door open.
Enter, then make a save as this achievement was bugged for me and had to replay this section.
Minor spoilers ahead.
*** Spoiler - click to reveal ***Examine the following things in this order:
- phone to listen the messages
- portrait next to the door
- cupboard door to get the diet coke
- Hobbs' body to get a closer view
- Hobbs' pocket to get the mints
- pinboard to get a closer view
- drawing at the bottom right
- Castell dels Sants on the map
Exit and the achievement should pop.
Maybe not all the steps are needed for the achievement but they are needed to progress the story anyway. When I reloaded the game, I followed these steps from the walkthrough, and this unlocked the achievement for me. If you unlock it with fewer steps please let me know and I'll update the guide.
Credits to Dang3R Gaming for the walkthrough.