strange sweet smell coming from air vents

During the winter, a strange sweet smell comes out of the air vents in the master bedroom. I hired a duct cleaner but the smell is still there. After a year of not being able to determine the cause of the smell, I cut some holes in the floor of the bedroom to see the duct system underneath the floor. The space underneath the floor really smells of that sweet smell. I have no idea where it is coming from, except that my neighbor's bathroom is underneath and possibly some air freshener is seeping through. Anyhow, there must be a leak in the air duct system because one, the smell is coming out of the air vents, and two, when I turn on the fan, a draft comes out of the floor holes. There is a main duct, from which three flex ducts come out. The flex ducts are attached with a "petaled tabs" arrangement, where each tab alternately goes in or out of the hole in the main duct. Do you have any suggestions on how to determine where the leak is and seal it? Thanks.