Watch Dogs features a number of Investigations that you need to solve by tracking down various collectibles. One of these involves finding QR Codes that have been split up and hidden throughout the city, most likely by DedSec. Track down all 16 of them to unlock a special mission, then complete it to receive the Read-only Trophy or Achievement.
Once you've found the area containing the QR Code, you then need to hack the correct camera nearby to line up the pieces so it can be scanned. It can be difficult to find the right angle to assemble the code, so this guide will show you exactly which cameras you need to use to get the job done.