Since Hunter has only started doing this for the past 6 months - I am wondering if it is not related to the new kitten. How lomg have you had the new-comer? He may have started it slightly before while sensing the preparations and new smells if you had been visiting shelters or whatever.

Did the vet check for urinary tract infection? They would have had to express a urine sample or withdrawn it with a needle.

Again, with a new kitty there, he may need for you to add another box (this was probably covered in the links Pif supplied)

Poor Hunter - just try and think like him and maybe you can see what he is trying to tell you - assuming there are no medical problems.

I think Resolve is a stain remover right? You may need to get some Nature's Miracle or some sort of similar enzyme cleaner to neutralize and eliminate the remaining scent.

Down the road, if it continues - there are always medications to try. How frustrating this must be!!