Since you have hardwood floors, It's more than likely the furniture that could be trapping the odors. I noticed that when ours park themselves on one couch, the smell can hang in there for days. I use Febreeze for fabrics on the furniture. The carpet doesn't seem to keep the smells but I vacuum all the time.
Also, how do your parents REALLY feel about the dogs? Is it love, or tolerance?
That might be part of the problem. They could be smelling something that just isn't there.