There are a total of 16 QR Code Locations and 1 QR Code Investigation Mission in Watch Dogs.
Finding all QR Codes will unlock the mission and completing the mission will earn you the Read-only Trophy / Achievement. You will also get a new weapon (Tommy Gun) as a reward. To reveal the code locations on your map you must unlock all ctOS towers. The displayed locations aren't very accurate though and you must find one specific surveillance camera and look at the QR Code from a certain angle. The correct camera is sometimes far away, or can only be activated from an elevated position.

All Codes are saved automatically when you find them. None of them are missable and you can still get them in free roam after beating the story. They are also a requirement for the "They Call Him The Vigilante" Trophy and Achievement.

TIMELINE:
#1 - Parker Square - 0:05
#2 - Parker Square - 0:39
#3 - Pawnee - 1:16
#4 - Mad Mile - 1:51
#5 - Mad Mile - 2:37
#6 - Mad Mile - 3:20
#7 - The Loop - 4:04
#8 - The Loop - 4:41
#9 - The Loop - 5:31
#10 - The Loop - 6:18
#11 - Brandon Docks - 6:53
#12 - Brandon Docks - 7:28
#13 - Brandon Docks - 8:06
#14 - Brandon Docks - 8:42
#15 - The Wards - 9:17
#16 - The Wards - 9:54
#17 - Complete the QR Code Mission - 10:55