Originally Posted by Rep
Scanned 2 hidden aliens in Story Mode
- In order to unlock this achievement you will need to scan the two aliens that are hiding inside human bodies. Click Y and hover over the human/object to scan. The first alien is near the end of Chapter 4 when you are leaving the muesum. Once you come outside you will spot several aliens and humans out on the street. The alien that is hiding inside the human body is to the left behind the police car.
The second alien that you will need to scan is located at the end of Chapter 7. As soon as you are finished with the flying section, you will come to some stairs leading down to the fountain. There will be several aliens and humans on the stairs. The alien that is hiding is at the bottom left of the stairs.
You can use chapter select if you miss out on the achievement in your first run of the game.
I found you can also obtain the achievement by scanning the first hidden alien, dying, and reloading the checkpoint. Upon scanning the alien the second time the achievement will pop.