You'll hear about the Saridakis death fairly early in the DLC. Before heading into the bank, you must talk to Saridakis's husband on the steps into the bank to trigger this optional quest (failure to do so will prevent the achievement from unlocking, even if you find all of the evidence).
Once inside the main part of the bank, there are four pieces of evidence to find.
1. The first is in on a computer in an office on the southeast side of the third floor. This is the same office where
takes you and solving that one will give you the password for the computer ("trustno1"). Or just hack it.
2. The second is on the computer in the executive offices on the fifth floor. You are supposed to read this email anyway as part of the main quest, which reveals which vault room you are looking for.
3. The third is in vault 209 (upper floor, left side). This vault is filled with poison gas, but you can turn it off by finding the value behind a hidden panel on the left side of the room right before the door into the vault. The evidence is on a pocket secretary in the back left corner of the room, mostly hidden behind some crates.
4. The final piece of evidence is a pocket secretary with a voice recording in the office you head into after getting into vault 204 (main quest). It's on the desk on the right (as you enter), underneath a few boxes.
If you talked to the husband to initiate the quest, the achievement will unlock once you've found the fourth piece of evidence.