The mission begins in the sewers. Use the nearest exit (marked with a red arrow on the map), making sure the soldiers don't see you as you surface. To accomplish this, you can check the army depot area of the street-level map to observe the patrols, or simply turn the camera south near the top of the ladder to catch a glimpse of the area before exiting the sewers. Be aware that both the nearby soldier and the soldier further to the south can see the sewer hatch at some point during their patrol routes.