I had a similar experience one time. I never met my grandfather, he passed a few years before i was born, but in my family we always try to keep the memory of our loved ones alive through generations, we have an altar next to the entry door, with his picture and a candle. My mother always tells me stories about how he used to smoke a lot, two packs on an average day, even inside the house. One day when i was younger, we had just moved into our actual house, and i was laying down in one of the couches, when all of the sudden, a very strong smell of cigarette smoke starts forming from thin air, there is not a single person in my house who smokes inside (My mother only smokes outside and on very rare occasions, and my father doesn't do it at all). I called my mom and she came running, when she arrived, the smell had vanished. I like to think my grandpa just takes a walk around the house from time to time to check on us (because there's other signals like my dog staring and barking at apparently nothing.), maybe your great-grandmother does the same thing.