I actually know someone whose neighbour had to knock on because their weed smoking was stinking out their house. It's quite surprising how the smell can start to penetrate walls and stuff. Once their neighbour told them they were obviously very apologetic. So, it doesn't necessarily mean your neighbour is growing it. They might just be a massive stoner. They also might be fine about it once you give them a heads up (obviously you'll have a better idea than anyone on here what kind of character they are though).