This is a 2 part achievement:During Act 1:
While interrogating Abel:
- "let him smoke"
- "I hope so"
- "[Let him sweat]"
- "You'll know soon enough"
He will then reveal a secret note in his shoe, and upon studying it, you will learn the call signal of "Rockingham". This knowledge is required for a dialogue option to appear later.During Act 3:
Upon entering the ship, you will sneak and be spotted by an enemy. When they call out to you, respond with "Rockingham", and make them leave and unlock the achievement. Keep in mind, this option is not available if you don't complete the interrogation properly in Act 1.
The following missable achievement can be seen in the video below at 0:55 & 11:30