Miles' Letter?

Miles tells you about his letter when he's lying on that table about to become a drone.

The message is in that cell that Johns was in, when you free them you can go back and get the message. When you go to meet Decker or whatever his name was after he get's free you get the option to send the message.

Accept the quest when you encounter Miles.
Later on in the game, you will return to the cell block, once the prisoners are freed.
Go into the cells, and in one of them, you will see the option to pick up Miles's letter. Pick it up.
Next Comm Station you find, after contacting Dacher, you will then be given the option to transmit the letter.
Achievement Unlocked!