On chapter 3, while you are in the Hawkins Mansion and go upstairs for the first time, you will have a few doors to go into.
For this achievement, go into the second door on the right hand side (The first door is broke and you cannot get in)
In there, simply press and hold
to go into investigation mode, complete it and achievement should unlock.
Heres my 100% Guides for it, go to 31:00 in my Playthrough 1 video to see where, when and how to unlock this!
Hope it helps!