Did you do a through job of washing the litter box? if so then the smell is in the plastic and you'll need to throw it out. I would throw it out anyway, if and when you decide to get another cat you'd need a new one anyway. Thats just MHO

I don't have a litter box anymore since I don't have a cat.

did you change your HVAC filter?

Yes

OK, my next suggestion would be to set up many bowls of vinegar around the room and leave them out for quite awhile. If that does not work try activated charcoal. You can buy the activated charcoal on Amazon, but the vinegar is less expensive so I would start with that. (i volunteer at cat rescue/shelter.). http://lifehacker.com/5814015/deodorize-a-room-with-a-bowl-of-vinegar

I'll try the vinegar - nothing ventured, nothing gained!

Good luck. We used it on objects and in relatively small rooms and it worked well, you probably need to use a lot more for a basement.

If that did not work, we saturated the object with Natures Miracle cat version; that usually worked well but it was the most expensive option so we used it judiciously.

When I got my cat from the humane society, they told me not to change her diet. I gradually changed it and boy did the waste stink. So change the diet, maybe ask the vet.

@ emma, did you READ the OP? Her cat passed away!

I can help but wonder if the [older] cat may have been missing the box. It wasn't obvious when the cat was still around but now...

I might suggest cleaning the concrete floor around where the box was located. Maybe any adjacent wall too.

Shame on me for laughing at mdln but it was a classic response

Hi Mike, I did that, but I'm wondering since concrete is porous the smell is in the concrete and can't be washed out.

The porosity of concrete will be a problem depending on how long the cat might have been missing the box. I might suggest several applications of those cleaners designed to neutralize cat urine odor. If the odor decreases after each application, you are heading in the right direction. I believe Nature's Miracle is supposed to work pretty well.

That's a great idea...thanks!

Had this problem in a client's house, cement floor, cat had peed all over it. I sealed the floor. That took care of it.

I'd probably do both the enzymatic cleaner - to get it as clean as possible in the first place - and then seal the cement so no further odors can permeate.

is it possible the cat peed in areas outside of the litterbox? I recall my cats would do that when they got upset - even peed inside my husband's dive bag because we went away for a weekend. I had to locate the source of the odor when that happened. These are only rare occurrences. They didn't pee on floors or carpets. I've 2 female cats and one litterbox. Outside of the litterbox there is no odor. Also if your cat was male, they have this territorial thing going and may have needed to mark certain areas. Check around the basement and more closely to where your intake vent is located. Maybe there's something there.

tibbrix - what do you use to seal the floor?

You can get concrete sealer at Home Depot, an extension pole and paint rollers to apply it. It's easy to do. Just follow the directions. Make sure each coat is dry, for example, before putting on the next coat. And since concrete is so porous, you're better off putting a few thin coats on than a couple of thick coats. The sealer will also allow you to clean up spills better.

Soaking the concrete with the enzymatic cleaner made for pet odors may help. I came home from vacation once and my house stunk like cat pee when I walked in and that isn't normal. Cat had peed over the edge of the box and cat sitter didn't clean this up. I moved the boxes, washed the floor then sprayed the enzymatic odor spray all over the concrete and left it to dry. That took care of the odor.

I can attest to the black light. My dog disappears when I get it out. Also used it when teaching to make sure the kids thoroughly washed their hands. They were funny. Looked at each other to see if they passed. You can check your guests as they come in.

Many things, incl. some detergents & cleaners, mold, tonic water will glow under a black light.

For the kids there is a special gel to pull of the hand wash trick. It is amazing what you see on your furniture and anywhere like walls. If you have OCD this is not for you. The urine I can identify as white.