I've had that happen before. Couldn't smell a thing myself.
Use a blacklight to try and identify where the exact spots are. In a darkened room use a blacklight and the urine spots should show up. Mark those areas with something (block, tab, something) so you know where to treat. Follow standard urine removal procedures on those areas.
If you miss one spot there will still be odor. Warn them it may take more than one treatment. Also no gurantees, especially if the animal is still there.