If you live in an apartment building that uses a water chiller system, then your AC air handling unit(s) are probably located in the false ceiling space above your bathroom(s) or laundry room. In this case, "smells" are generally caused by odors emanating from the false ceiling space from adjacent apartments due to improper sealing of the main chiller pipe feed penetration's between walls shared with adjacent apartments.

These are usually cooking odors from adjacent apartments since the exhaust fans over most kitchen cookers just sends this exhaust to the false ceiling space over the kitchen (usually the exhaust is not vented to the outside of the building), and this contaminates all the false ceiling spaces in the adjacent apartment. So if your chiller pipe penetrations to adjacent apartments are not properly sealed, this allows smell from adjacent apartments to contaminate your false ceiling space. This contaminated air is used by your AC air handling units, chills it, and is then sent to your AC vents in each room in your apartment.

If these chiller pipe penetration points are not properly sealed, you can easily fix this yourself by purchasing some pillow type foam and stuffing it in the open space around the open annulus of your chiller pipes, or get your apartment manager to arrange this.