You obtain this early in the Arson Desk mission "Nicholson Electroplating" (this is case 25 of 26, so towards the end of storyline). As you inspect the site of the explosion, on the left side of the site there are 2 uniformed police officers that will call your attention to come and see something. One of them will want to show you a METAL LOCKER with a briefcase inside. This is NOT the clue you are looking for.
The second officer points the right clue (metallic bin-like cylinder) which once inspected, you just have to align the letters for the clue to count and achievement should be yours.