Dishonored: Death of the Outsider

This one is easy. Use semblance to take the face of a veteran guard, one of the red uniformed ones. Go up to another guard, one of the junior ones in blue/green. It's not really a salute; it's more like a wave.

Sometimes they do it, sometimes not. If they don't try talking to them, or use a save to reload and try again. It seems to be totally random.