If you're going to try sealing any cracks, turn off all your fans, the AC and close all the windows. Take a lighter (or preferably a candle) and run it along the cracks you find. If the flame flickers, voila, you got a crack with a draft---seal it. It helps to do this along all windows, along all corners of the walls, etc. Ask your neighbor if she still smells smoke after you've sealed all cracks, and if so, test the AC ducts with incense, and have her standing by--don't tell her what you'll be using, just ask her afterwards if she smelled anything, what she smelled and where it was the strongest in her apartment. If the leak's in the ducts, you can't fix that, but at least you've pinpointed what to fix for the maintainance folks.