Can easily be done in the first mission, simply use a distraction technique to lure them over.
In the first mission, Safehouse, whistle/call (SELECT button) an enemy and hide in the shadows, when he's inspecting press SQUARE to do a takedown. I got this when an enemy was called over and was inspecting one of his unconscious colleagues.
I assume it can also be done by switching lights off or by using the sound gadget.