This is one of side cases, which you can solve in Observer. You can unlock it after completing the initial part of the game, that is after investigating the crime scene in apartment 007 and leaving it. That will unlock access to a large portion of the block of flats.
For the case in discussion you must head to the second floor of the building and reach apartment 205 (use the maps on the walls if you have troubles reaching that place). After locating apartment 205 you will notice a faulty shutter, which makes unpleasant noises. Crouch down and walk under it in order to get into the apartment (before you enter there you can pick up the collectible picture lying next to apartment 205 entrance).
Inside apartment 205 you will make a lot of disturbing discoveries, leading to the conclusion that you have found yourself in an organ dealerís apartment. Apart from another collectible picture there are a lot of objects to scan with EM Vision or Bio Vision. Namely, these are: two livers, a heart, skin, biomass and a computer (you can read all e-mails).
The most important finding is shown in the picture above, and that is a couple of photos. Examine the bottom-left photo to reveal the code 7441, which gives you the access to a particular cellar.