Cigarette smoke can permeate all of the clothing and even the suitcase or bag you took with you to the unit. If you have any of those chothes that you have not washed yet, it can still be around. Even on papers or in your hair (I'm sure you've washed it by now.)

Also, it can still be in your sinus cavities. I don't think you are hallucinating. I smelled cigarette smoke for a very long time after I left my mother's home after taking care of her before she died. Her home was so full of cigarette smell that I smelled of it for weeks. I even had to wash the inside and outside of my suitcase.

When I was there, I had to go outside on breaks because I would get headaches.