First clue is a damaged phone which can be interacted with, and viewed. This device only spawns in the Arinobu District (HUB World), the phone changes location every scene change, if the phone is not present then reload the scene.. Common locations are as followed:

• In the Alley on what appears to be an AC unit.
  
• On a stand going the long way around to the Arinobu Station.
  
• In the Alley on what appears to be an AC unit(2)
  
• In the Alley on the floor next to a trash bag.
  
• Next to a trash bag in front aside the Alcohol Store.
  
• Past the player home where the pink sign is, found on top of a cardboard piece.
  
• On top of a TV in the Requests bay.

The Phone
Upon inspection of the phone:

Given the context, the text could be traced to Base64.
By translating raw text to Base64:
Your presence is requested. Meet�below the Roppongi Saitama Group Research Facility on ި��&�Zݫwi���) to receive contract details.
Bring a radio. Expect 1611 kHz

Destroy device upon receival of this message.

Using the information obtained from the raw text decryption, we can assume the radio location. If you remember, the player is released from the very facility, and only facility where "underground" takes place. The answer to our question has been under our noses.
Back to the Past
After starting a new game, make your way through the intro (as usual), stop and wait for a period of time in front of the radio. The radio will eventually spill the "contract details" as seen in the damaged phone. A sequence of numbers will be provided.


1 18 9 14 15 2 21 5 11 9 20 1 11 21 8 1 9 19 8 1 3 15 4 5 9 3 8 9 7 15 8 1 3 8 9 19 1 14 7 15
This number sequence lines up with Letter Number Code. Taking a look at the result:
ARINOBUEKITAKUHAISHACODEICHIGOHACHISANGO

Closely determining what stems from the text, we'll see that the string includes the word CODE in English. What follows is a sequence of numbers, five digits of course. These numbers are in Japanese.

ARINOBUEKITAKUHAISHA "CODE" = ICHI GO HACHI SAN GO /15835

Preparations for using the Laptop and concluding the search:
You will unlock part of the map that is not normally accessible, you will be met with heavily armed resistance, Arinobu will no longer be a safe area.



If you were expecting a laser gun? High frequency blade? Nuclear button? Permanent character upgrade? Well, be disappointed then. If you were looking for lore? Well this is a dream come true because it would have definitely been worth your time! It's the most direct character related lore in the game, and the most exciting!

That about wraps up the experience. You will continue to be attacked until you reload a save or begin a mission. The Laptop can be activated so you may review the contents at any point.