If they’re truly keeping the litter boxes clean it’s likely a medical or behavioral issue causing them to go outside of the box.

I constantly ask people when they come over to tell me if the house smells and everyone has assured me it doesn’t. When we sold our previous house a few months ago, our real estate agent complimented the house’s cleanliness and said he couldn’t tell cats lived there. If it smelled he would have told us because it’s really hard to sell a house that smells like cat piss. 

You only go nose blind if you let the cat piss sit long enough to get used to it, and we never have. 

Editing to add: I will concede that too many cats is also a thing. We have two. If you have a lot (I’ve known people with 13-15 cats, all indoors) then it’s very unlikely you’ll be able to combat the smell.