Richard,
If the fan is not on, winter, not working, etc. Cold draws to hot, (at least thats what they said in insulation class) so naturally cold air will enter through the fan "when it's not on" and into the attic area along with any expelled sewer gas. How much? how often? to many variables.
So Jerry is correct but I thinks if the owners where smelling gasses they would of had it checked out long ago. Or they are moving because of it.
The buyers could always install a pilot light on the fan and make it a flame thrower
