Walls and baseboards 1 ft up, furniture, floor vents, drapery, bedding.
Lots of hiding places for odor.
Carpet cleaners tend to limit odor treatments to carpet because, I suppose, we are used to be called to clean carpet, or at times other floor surfaces. As brought out and shown on video, dogs and cat's don't just "deposit" on flat flooring surfaces.
Most odor removal "failures" come from missing a contaminated area, whether it be carpet cushion underneath, or Lee's above mentioned areas.
I had one that involved cat urine on top of kitchen cabinets.
That is why, as a landlord, I will accept dogs, but never cats.