Chapter 2 - Section "You have the Intel?"
In the second chapter are 4 alarm systems. They are small, red consoles. Enemies will activate them if they spot you to call in reinforcements. For this trophy, you must prevent that they activate any of these consoles. The best way to do this is by hacking the alarm systems with the OWL. Press L1 while aiming at a console to hack it. After that, enemies can no longer use it and you are safe.
Do this to all 4 consoles and complete the main objectives in the "You have the Intel?" section to unlock the trophy.
Also be sure not to hack the security mainframe. That's the console on the second floor of the radio tower (last objective, where #4 is in the video). Hacking it will void the trophy. If you trigger an alarm, just restart the checkpoint and deactivate it next time.