You can find 16 QR codes around Chicago. They are marked on your map for you as you clear the ctOS towers. Once you find all 16, you'll receive a special mission, which you can complete for an achievement/trophy and the Gangster Tommy Gun.
Below, I'll provide images to help you find each QR code, first with a picture of the map and then with a picture of the view from the camera that allows you to see the code, along with a written description. I've organized the information based on the general region in which you can find a given code.
You can find a code in a boat yard, located near the north side of Brandon Docks on the large island. The code is on a building, and you have to head far enough back from it that you can find a camera on another building that looks out over the large metal building with a Racine sign out front.
A code is located not far south of the large island that is positioned along the north side of the Brandon Docks area. The code is spread across three objects, so you need to view it from a camera mounted on the corner of a brown building near the elevated railway tracks.
Near the southern side of Brandon Docks, closer to its west edge than center, you'll find a code. It is on a distant building that you can view from a camera mounted on the corner of a gray building near one end of the bridge.
Near the south end of Brandon Docks, toward the eastern side, you can find a QR code near a large area with construction cranes. The camera you need to view it from is located so that it looks out over a facility storing several rows of portable toilets.
Look near the central north portion of The Loop. There are four buildings there, close to the marker on the map. The southeast of those four buildings has a camera on its side, near the street. It's not particularly high on the building. From that camera, you can see a QR code nearby that is easy to miss.
Near the west side of The Loop, basically due west from the western hideout in that area, you can find a QR code. It is located high on a building on the west side of the river. You'll need to climb your way up the cameras along a side of a skyscraper near the orange iron construction that supports elevated train tracks (near the street level, and also near phone booths). It's easiest to get to the camera that will point in the direction you need by heading down a nearby alley, climbing onto a truck, then heading along a grated balcony to reach a perspective that lets you see the camera so you can hack to it. This is one of the trickier QR codes to find.
Near the southwest side of The Loop, across the river from the Bunker hideout that serves as your base of operations throughout much of the campaign, you'll find a QR code near the homeless camp under the overpass. It is painted along a separate group of support pillars. The angle you need to view the code from is only attained by crossing the river and then looking for a camera on one of the bridge's support columns on the opposite bank, along the side of a grassy slope.
Not far from the far south point in The Loop, near the L-train station, you'll find a QR code. It is easiest to see the code if you climb up to the higher ledge at that station and look out toward the northwest. The code is on several buildings in that direction. The angle from which you should view it is achieved by hacking into a camera on a building that is visible from the raised boarding platform.
In Mad Mile, just a few steps south of your hideout along the north side of that area, you'll find a QR code on a building across the freeway. Look to a gray building on your side of the street. Travel to a lower camera along the right side of its lower wing, then from there you can travel to a camera higher on the building that looks across the freeway at the perfect angle to allow you to scan the QR code.
Due south from the north hideout in the Mad Mile area, not far north of the region's south border, you'll find a QR code. Look for the large gray building with a sign advertising LT at the top. Jump to a low camera there, above the ATM machine, then to a camera on the next building over. From that camera, you can look out and see the QR code on a high building across the street, to the right of a car advertisement.
Near the center of the Mad Mile region, you'll find a QR code high on a skyscraper at the corner of a four-way intersection, near the elevated train track. Look for a castle-shaped building (it's brown, at the center of a square, and rather stands out), then look for a gray building across the street that points toward the high building with the QR code.
Not terribly far east and slightly north of the western hideout in the Parker Square region, you'll find a red building with a sign advertising "Chicago's First in News," at WKZ TV. On the face of that building, on the side opposite the ATM, a camera is mounted up on the second story. From that camera, you can see the QR code on a distant building across the block.
The southern of two QR code spots in Parker Square is painted on the side of a building across the street from a building with a banner advertising Gorman Heights. Head down the street with the "Protect Our Children" illustration painted on a wall along the right side. On another building on that side of the street, you can find the camera you're looking for. Pivot it left to see the QR code from the proper perspective.
Not terribly far north along the road from the motel in the Pawnee region, you'll find a QR code on a billboard positioned over the entrance to a tunnel. There's a consruction site nearby, and one of the cameras near its entrance is located in the perfect position. Look through it to see and scan the code.
Near the northwest corner of The Wards, not far northwest from the hideout in that vicinity, you'll find a dam facility. From the nearby dock, steal a boat and ride out toward the closed dam. You can find a camera on a platform that looks along the dam, toward a position near its corner where the code is painted.
Near the south side of The Wards, not far from the L-train station, you'll find a QR code on a building not far from a Defus Tru store that sells hunting supplies. Across the street from that building, you can hack into a camera that looks out over a rooftop and at the area's QR code.
Once you have found the 16 codes, you'll be able to begin a final investigation of the matter. In Mad Mile, you'll find the point where you can begin your bonus mission, on the side of a building. Once you accept, you'll receive the first objective.
Find a way into the hacker's apartment.
You'll need to solve a hacking mini-game.
The mini-game is in three dimenstions. Point the camera down to see the base, which is where you should start. You'll need to turn elbows and other pieces to send a current up to reach several lock icons. Start by unlocking the upper right, then the top, then the bottom and left and finally the center locks, in that order.
Upload the DedSec virus.
Next press and hold the button indicated to perform a hack. Then the mission is complete.
NEXT: Missing Persons