These are non-respawning enemies, usually on rooftops, sniping you from above.
There are 5 of them. 3 in Jericho Downtown, 2 are guarding the entrance to the Cathedral of the Spark.
You need to kill the operators (not the turrets)
You can kill them with your drone (but targeting them is not easy) or melee. For melee, you need to find a way up there. You will probably get to all of them through natural progression and side exploration, but one is a little tricky - the one on the left of the entrance to the Cathedral (see below)
Sniper 1: Downtown Jeriho City, the rooftops, just outside the Seaside Court, leave via the upper floor exit and cross a small 'bridge' (you can only do that after killing Little Johnny and Brother Eli in Port Nixon and talking to the guys downstairs in Seaside Court.
Sniper 2: Downtown Jeriho City, on a ledge of a building near the entrance to Babylon Gardens and the Cloud 9 Bar.
Sniper 3: Downtown Jericho City, in the dark ruins of a building (just off Gideon Plaza, through the gate, on the way to Gateway Bravo)
Sniper 4: Cathedral of the Spark, right of the entrance, ride the exo line up and you'll be able to kill him.
Sniper 5: Cathedral of the Spark, left of the entrance. Progress through the location, draining the coolant as you go. How to get to the sniper: jump to a ledge with a jammed door right below the room with the Med Bay. Explore the new area and you will soon get to him.