There are 63 Echos in the game which are left behind data snapshots of events. These are collections of images and sounds, collected by security cameras, phones and more. The agents gear then correlates all that data and creates a projection into the world, looking like a cloud of orange dots. They tell you small stories about what happened in this place in the past. You get more background information through them. Most are placed in the open world, but some can also appear inside missions. There are no known echos in the Dark Zone.
The echos appear in fixed spots in the world. At first you see nothing until you stand in the spot to initialize it. Then you can load it with a button press and the orange dot data appears around you. The echo has a certain limit, a border around it, which will end the echo if you step out of that area. You can also collect the echo, once it's up. So don't forget that if you are after all Collectibles.
In the orange cloud of dots, you can find some white ones, four short lines forming the corners of a square, floating in the air like the rest. By getting closer to these you can get additional information about the echo or the specific thing you are seeing in that one spot.
Once you leave an echo, you can return and activate it any time again as you wish.
You can find detailed map locations on the Interactive Map.
This is an easter egg portraying the Shawn of the Dead scene that does exactly the same. From 9th Ave, head between the buildings east between 23rd and 22nd. Just when the walls open up to the alley proper, you can find it to your right behind the fences. A bit to the west is a phone ringing.
April Kelleher: 01 Riot
In the middle of 23rd St, between 8th and 7th Ave. In the center of the block right on the street.
Rogue Agents: 05 Retaliation
In that small space north of 47th St and west of 10th ave. It's a construction site and a large truck with an orange container is parked at it's eastern boundary. The Echo is under the gruesome display with the excavator and the JTF soldier.
General: 22 Shepherd
Right outside the church on 45th St, between 9th and 10th ave. It\s almost in the middle of the block, closer to the east though.
General: 03 Interview
Next to the Dark Zone Entrance on 36th Street, right in the crossing with 6th Ave.
Michael Dufrane Part 3
Part of the side mission to find Michael Dufrane. Simply do that and you will be guided to this spot.
On 3rd Ave, between 20th and 21st St in the middle of the block and the road.
From the intersection of 42nd St and 9th Ave, go west and into the first open building north of the road. It's sort of two parts built together, with the western one having a prescription counter. Stay on the east side, go to the back (north) and into the second back room (eastern most) to find the echo.
General: 14 Protest
In the middle of 33rd St, between 1st and 2nd Ave, pretty much the middle of the block.
General: 04 CERA
Just outside the Base of Operations. Go left, and you find it at the northwest corner of the Madison Square Garden block next to some tents.
General: 10 Power Plant
This one is inside the Mission WarrenGate Power Plant. Simply complete it and in the master control room at the very end where you get your reward, there in the corner is the echo.
General: 21 Snowball
Look at the three loops on the map in the lower right. In between them are 4 building that each look like two plus signs merged, 45 degrees turned each and forming a square. The echo is in the middle and the a bit to the north inside that square. South of it is some sort of Christmas market with stands and where you can find weapon parts.
General: 6 Cleaners
On 21st St, just east of 6th Ave is a parking lot north of the street. The echo is in the northwest corner of the packing lot near the white van there.
General 25: Hostile Takeover
Between 45th and 46 St. There is a parking garage that is level with the street. The echo is inside in front of a police car.
Miracle: 2 Fearless
In a construction site between 47th and 46th St, just west of 1st Ave. This is also an encounter for the medical wing, hostage rescue. Climb up the ladder to the south, then go up the stairs and you find the echo near the southern edge.
Lexington Event Center: After going through some double doors, you see a restaurant where the path goes to the back left wall. The echo is in the center of that room.