Mission Footprint Zero Achievement
Mission Footprint Zero10
Completed a single player mission undetected
You can attempt this achievement on any of the missions after the Prologue. The first mission, Safehouse, is one of the easiest and shortest missions to get this on. Make sure not a single enemy shoots at you or rings any alarms. The Detection Meter (half circle that fills up when they see you) above their head may never turn red. If it does, you have been detected and must to restart from your last checkpoint! If the half circle turns fully white, you still have a chance to quickly take them out.
I suggest doing this on the Rookie difficulty, as the enemies reaction time is much slower. Also make sure to hide any bodies you knock out so they cannot be found. Like any good ghost operative, use cover often and try to find alternate routes around each area you encounter. You can replay campaign missions at any point by accessing the SMI in the Paladin. You'll be able to keep all the gear you've already purchased, thus making this achievement much easier.
Be sure to check out THIS THREAD to find a video of a successful undetected run on Safehouse.