This achievement requires you to Find the blood stained knife on your first sweep of the hit and run at Ray's cafe. Ray's cafe is the first location you go to during the traffic desk case "A Marriage Made in Heaven". Once you get to the crime scene, go down the left side of the cafe into an ally and as soon as you enter, you will notice two trash cans.
Approach them and press
when you hear the clue noise prompt. The achievement won't pop right away, so don't worry. It will pop a bit later in the story when you need to head to the morgue.
It is very important that you find the knife the first time you visit the scene as that is the requirement for the achievement. If you forget, you can always restart or replay the case from the main menu.