When we had an indoor cat it seemed the litter box always had some odor of varying degree's. We added baking soda to the liter to try to help with the odor. Not sure it helped. Masking the odor helped the best probably with a plug in fragrance type thing.
My in-laws have one of those automatic scoop cat liter boxes. They say it has no odor but ummm whatever. That back bedroom where they have the liter box smells BAD!!
I loved our indoor cat but I loathed the cat box situation. Didn't help that we didn't have the best place to put it. Oh, the best place was in our last house. We put a cat door in the garage door into the garage and had the liter box out there. Best thing ever!! Can't do that here. our cat since went loony. She kept messing (poop) which is actually easier to get the smell to go away, on the rug by the door. We got rid of the rug and then she started doing it in the guest bath tub. We took her to the vet, tried all kinds of things, trapped her by the liter box, all kinds of things. Finally I had HAD IT after she messed on our sun room rug and threw her outside. About two years later she is the healthiest cat ever! She was huge and now she is slim and trim, very active. My daughter does bring her in to feed her twice daily and show her some love, but she just stays in our yard and roams more than i'd like her to but it is what it is...she ain't coming back in!!
I also told my husband.....as great as cats are, that did me in for ever wanting an indoor cat again. Also saw my sister in laws cat go cat pee crazy in her spare room and that was a nightmare to fix. hahaha....off topic sorry!!