One thing to bear in mind is that no matter what cleaner you use to remove the smell, the ammonia in cat pee is hard to remove. This actually attracts the cat and therefore people (like me!) find that once their pet has peed outside the litter box, they keep going back to the same place to pee.
Maybe you should watch your cat to see if he/she is being attracted to the same place. If it is, then maybe it was cat pee after all. If its not, then it's your carpet.
If you find out it IS pee, I suggest you search online for some products that will help remove the ammonia. I know that lemon works, but that will bleach your carpet.
Good luck!