I've been dealing with cigarette smoke smell the past couple of months. It's a common paranormal phenomenon, evidently.
A psychic told me it's a land spirit, he's harmless, is infatuated with me, doesn't want to go into The Light cuz I'm his dream girl. I'm not flattered!
She said his spirit guides will be coming to fetch him and take him into the light soon. Not soon enough for me!
Reading opinions on the OP's dilemma, I can debunk mine:
I don't smoke.
No one I know smokes.
The neighbors in my building and the building next door (only four units each) don't smoke and are seldom home as they travel frequently, often gone for weeks at a time. One reason I like it here - very quiet.
It's a 100 yr old building and the only thing 'shared' is a solid brick and plaster wall with the adjoining apartment. No upstairs, no basement, not on the street side of the building.
Hardwood floors were newly refinished when I moved in years ago. Ceramic tile on bathroom floor. Kitchen linoleum is one layer over hardwood floor.
When I've smelled the smoke strongly, I've gone outside, front and back, to see if anyone was there. Nope. And there was NO smoke smell outside. Only inside.
Apartment manager and handyman don't live on site. It's hard to get them to show up even when tenants ask!
I might know who it is, though. Many years ago there was an old geezer, alcoholic bum, who'd sit on the curb smoking. He always gave me a big, gap toothed, tobacco stained smile. I was civil to him, nothing more. Cops took him away eventually when he beat up his wife. Both died years ago.