Oh dear. There has to be some cooperation between yourself and the neighbour to track the hole or holes where its coming through. assuming its a terraced house for example the stairs may well be where the hole is in a cupboard either in the kitchen or under the stairs where they join the party wall. In my house its a cupboard where the idea was that a larder with an air vent in the top was built under the rise of the stairs. Sadly the designers made the void to the outside shared between the two houses, no doubt for cheapness.
One person in one house with something smelly t trying the various places while the other one sniffs in those places the other side of the wall in your house. The basic principals of this are no doubt the same for flats and other buildings. Brian