This is in the later part of the village level (after you chase Juan from the church). You will notice you are in a large, open-air market with wooden stalls. Just make it through without killing civilians.
If you are careful and make sure you can see someone shooting a weapon before killing them, you should be fine. Most of the civilians cower down, helping you not kill them. A lot of enemies take cover behind stall counters where there may be civilians, so I would hold off on grenades during this part.
This will unlock at the end of the market area (in mission) if you did not kill any civilians.