After you obtain the wooden statuette (drain the bathtub in the upstairs bathroom of the main house) and solve the shadow plinth puzzle in the main hall of the main house, a new pathway will open in front of the shadow plinth. Head into this pathway and you will soon come to an area called the drawing room. There's a large stuffed deer in here. Head to the wooden table in the drawing room. You should find some psychostimulants on this table. Take it.

Psychostimulants make item detection much easier. You can choose to save the psychostimulants for later consumption, or use them now and unlock the achievement.