(Cont.)
I find it very hard to believe that the smell would drift downwards through their floor, through any gaps in the walls (which shouldn't be there) and then up through your flooring, it would be more likely to leave through any airbricks you have below floor level so check your airbricks are clear just in case, this might actually solve the problem.
Just another thought, Chuckfickens asked if you have any teenage kids, obviously that's an area to explore, but don't forget, even if your kids deny smoking, they and their clothes will smell of smoke even if they are hanging round with a gang of kids who do. I know this from personal experience. Our teenage daughter was coming home stinking of smoke yet she still swore it was just her friends who were smoking. Also, when our kids were young we used to take them to our mam's for a night, and when we brought them home we had to wash every bit of clothing and bedding to get rid of the smell!
Good luck finding the source!