I can't be sure but I think it is coming in through the ventilation system. Even though we all have our own duct work, heating and cooling systems the smell must be getting into the walls and then into our duct work. I always know when they're smoking, and if the A/C kicks on during that time - ufda!
We hadn't anticipated this as the smoking neighbor wasn't here when it was our grandma's place. In fact she didn't have a smoking neighbor. These people are new. To make it worse the smoker is home all day too.
I've tried putting dryer sheets in all the vents but that really hasn't stopped the stink. I figure our next step will be getting some expanding foam to fill all holes under the sinks and such. Then have a conversation with some professionals to see if there is something we can do to block the smell in the duct work.
I'm determined to make this work for 2 or 3 years so we'll figure something out to keep the smoke out.