1) Enter the tallest apartment building structure type(in larger cities like Cherno, Dubky).
2) Climb the ladder in the elevator shaft to the apartment floor with open doors (lootable).
3) Take the stairs down (past apartments with closed doors, no loot)
4) At the bottom (2nd floor) you will find that the stairwell to the ground floor is broken, continue down the broken stairwell.
5) You should now be teleported into the closed room adjacent to the stairwell.