Interrogating neighbors is an optional objective, but you should not skip it, especially that you can do that while going to the terminal needed for finding the woman who called the victim.
In order to achieve that optional objective you must use intercoms on the doors of neighboring apartments. Not all residents are at home – you can talk to someone in apartments: 008, 006, 005, 003, 002, 009 and 016. During some interrogations you may face some dialogue options, but they do not have much influence on the outcome of the conversations.
In order to proceed with the main mission objectives you must return to the room where the guard was. However, you cannot walk through the main door leading to the staircase, because it is locked. Instead, go through the large hole in the wall next to apartment 008. Your goal is to reach the closed door with a code panel next to it. Interact with that and use the hacking ability to learn the code – in our case it was 0446.
Walk on forward and follow the tight corridor toward the place where you started exploration of the building (you can call by apartment 017, but that is optional). When you enter the guard’s office, look around for more syringes with synchrozine and check the guard’s computer. You can browse through all tabs, but the most important is the one with documents. Open the document visible in the picture above, which shows the list of first floor residents, and you will find out there are two women with N. H. initials.
Upon leaving the room you will be stopped by the guard. You must talk to him again before you can go to the apartments of interest.
Note – there is a picture hidden behind one of the monitors in the office. This is one of 69 secret objects that you may want to hunt for in the game.
The two apartments are on the first floor of the building – use the stairs to get there. One staircase is at the end of the corridor where the guard’s office is, but in this case it is better to take the other staircase. For that, go across the large courtyard again and take the same passage that you went through while going to apartment 007.
After reaching the stairs, go to the first floor. Apartment 106 is closed, but you do not have to go inside, as the mission objective gets updated when you reach the room 104. Open the door leading to the corridor with the apartments 101-104. Walk through the corridor and enter the apartment 104.