This is unofficial advice given neighbor to neighbor; accuracy is not guaranteed, and any work on a unit should be done by licensed professionals with proper permissions from the building management.
There are many avenues for smoke and odors to enter your apartment. The two main ones are in and around your bathrooms and in and around your heat pumps.
There is a very large chase behind your bathroom’s wall with all the fixtures. The void is about 2 to 3 feet wide. On the second floor of your apartment, typical walls are brought flush with the main beam which runs down the center of your apartment. Smoke may enter around this beam. Additionally, smoke may enter through your bathroom vents. Good back draft dampers may be installed to prevent smoke from entering via your bathroom vents.
Heat Pump Chase:
Another sore spot is the area around the heat pump. There is a chase which runs behind your heat pumps which contains the water pipes and the condensation drain. These pipes don’t run vertically the entire six floors of the building but instead step outward as the apartments on the higher floors have narrower exterior walls than the floors beneath them.
There are two areas to seal. One area is in and around the heat pump. This may not be sealed properly. If the heat pump is not original, the installer may not have done a good job sealing the holes to this chase. When your heat pump fan kicks on, it can draw air from your neighbors below or above you.
The other area is again in and around the main beam down the center of your unit on the second floor. In this case, to seal this area, you would need to do one of two things. Either seal the air infiltration around the chase or seal the beam around the wallboard. The former is the ideal solution.
The last area of smoke infiltration is from the front of your unit around the center beam and around the front door.
We’ve been fortunate in that we haven’t been bothered by excessive smoke or odors entering our unit. But when I do notice something, it’s often coming through the fire corridor. Were I to try to block air flow from that corridor, I’d weatherstrip around the 2nd floor fire door (being careful not to restrict its use in any way), and then turn off the lights in our loft and see where around the beam I could see light from the corridor shining through and take steps to seal those areas.
In addition to the heat pump chase and the bathroom chase/vents, there is also the vent in the kitchen to consider when thinking about backdraft dampers.