I know the situation you are in! That is what my kitty Snuggles was doing, although his was from not liking the covered box we got. Glad you are going to have the vet check him out, hope all is ok. You might talk to your vet about possible ideas/solutions if it is behavioral. He could be upset you haven't been home alot and he is trying to tell you. My vet said there is a medication route but I really didn't want to do that if I didn't have to. There is really good stuff that is vet reccomended too, called Feliway. It's a little expensive but it helps with behavioral issues and urinating outside the litter box, etc. There is a spray and a plug-in. You can even get it at PetSmart. Here's a link if you want to check it out and read about it.
http://www.catfaeries.com/feliway.html
There isn't a horrible smell from the urine though? I knew when Snuggles did it because I could smell it, cat urine is usually a strong smell, especially in the carpet! We sure found that out! lol And you are doing all the right things to get it out!
We tried and tried and used tons of mixtures of things to get it out when he did it at our other house.
Hope all checks out ok and you figure out whats going on. Keep us posted.
