Since there isn't a guide for this as of yet, I figured I would point out that this is story related and can't be missed.
In the second mission of the DLC (at the very beginning...as soon as you take control) you are to stab a guy in the back. Do this and take about 10 steps towards the objective marker. You will be thrust into a cutscene and will end up interrogating a spotter on the rooftops. At the end of the cutscene, this trophy will pop.