cigarette smoke and my non-smoker mom, opinions?

This is about my mother and before I go into the situation I quickly want to tell you about her father. She had a very special, loving relationship with her father that was, unfortunately, cut very short. He died in a car accident when she was only 6 years old...
I was talking to my mother about some new things I had been reading up on and got onto the subject of some strange things going on with me recently. This made her think about something she has been experiencing which is the "here one minute, gone the next" presence of the smell of cigarette smoke. She is not a smoker and never has been. She told me that ever since about 6 months ago whenever she was working late (which is almost daily) and literally the ONLY person in the whole building she would smell cigarette smoke so strong it would almost make her nauseous and then it would completely disappear. Sometimes the same thing would happen while she was home alone. Now, when she first told me this I kind of worriedly asked if she was sure she wasn't smelling something like sulfur (the thought of my mother alone and regularly watched by something unseen that brought with it the smell of sulfur made me very uneasy), but then she said something REALLY interesting..."You know, my dad was a smoker until the day he died."
After I heard this, along with much relief, all I could say was "oh my god, it absolutely has to be your dad!"
Nothing else has happend other then the smell and she has never really felt afraid, so it doesn't seem like it would be anything other then her father or at least anything meaning harm. Maybe hes making sure shes ok since she is alone so much?
I feel pretty certain, but I would love some opinions!

I believe some spirits are associated with fragrances, so it is probably her dad checking in on her. Perhaps there was something left unsaid, she should speak her emotions or thoughts to her dad the next time she encounters the smell.

and welcome to ATS

The first question I would have to ask considering this is happening at night is the buildings heat turning on?
After everyone leaves, and the sun goes down the absence of both body heat and solar heat may be causing the building to turn on the heating system. When a heater first fires up, it often has a very strong burning odor to it.

shes positive it wasn't the heater and she can smell cigarette smoke from a mile away. she said he smoked lucky strikes and she knows the smell well. talking to it was a wonderful idea, i told her to try that the next time it happend, i guess we will see the outcome of that. thanx for comments!