There are a total of 16 QR Codes that need to be scanned within the game before you are given the final investigation and can complete this achievement.
To have the locations appear on your map, you will need to progress through the story and unlock the ctOS Towers, but you can scan them early if you follow the guide.
1 - [0:05] - Pawnee
2 - [0:40] - Parker Square
3 - [1:08] - Parker Square
4 - [1:48] - The Wards
5 - [2:46] - The Wards
6 - [3:29] - Brandon Docks
7 - [3:56] - Brandon Docks
8 - [4:40] - Brandon Docks
9 - [5:24] - Brandon Docks
10 - [6:00] - The Loop
11 - [6:43] - The Loop
12 - [7:30] - The Loop
13 - [8:25] - The Loop
14 - [9:01] - Mad Mile
15 - [9:42] - Mad Mile
16 - [10:21] - Mad Mile
17 - [10:59] - Mad Mile (Complete Investigation)