Googling "brain tumor phantom smell" only game me general information and nothing about how it's experienced. Do they come and go, or are they constant? Is the onset sudden? How frequently and how regularly do they happen? How long do they last (hours? days? years? minutes? seconds?)? Is it possible for a phantom smell to be experienced in one particular place?
One scene I was thinking of is my character experiencing the phantom smell outside the bedroom, and just outside. The smell disappears when he goes to the bedroom, and reappears when he comes out. In another scene, he's doing something, maybe on his laptop, when he has the phantom smell experience again, but just for a few moments. In both instances, it came and went suddenly.