From my limited knowledge of this system I would say that the smell is being sucked into the return air from the upstairs.
The condensate tube is downstaris with us, and it drains directly into a floor drain. I've looked at the floor drain and can't see a problem there with buildup, etc. So that said, it leads me back to the first sentence? If this is indeed the case, how do I combat this? (short of telling the tenant to clean up, which as you can imagine would go over like a lead balloon)