How to Acquire
As this is one of the more difficult accolades to collect, it is paramount that Corvo takes alternative paths to the targets and/or objectives of the side missions. Common methods of staying undetected are to possess animals or people and to find alternative paths (they are generally higher, or lower, than the enemies or the target themselves).
However, this achievement does not require Corvo to stay hidden from the assassination targets, and as such, his presence can be known to them without affecting the outcome of the achievement. Keep in mind that assassination targets can sometimes alert other enemies, so it is best to avoid doing so. Furthermore, Shadow can be easily acquired in the same playthrough if enemies are not alerted.
If Corvo was not detected, the option will be checked off on the end mission screen. If Corvo did not kill anyone, the corresponding space will be checked off as well.