Case 3 Achievement:
Towards the end of case 3 you will be locked up in the bad room and the game will switch to a new character(monkey). After many cutscenes you will start with monkey in the office corridor, from here go straight ahead through the green door and collect the clue detecting powder on the desk. Now go back down the corridor where you started from and spray each of the photos on the wall and each one should reveal a kiss mark, after the third one it should give you this achievement.
If you need more help with this you can follow my guide below which will take you through it starting with this part: