Do you have a new puppy or a cat who sometimes happens to tinkle outside the litterbox? I have the latter. Normally, I am around when it happens, so I clean up right away. However, accidents do happen when I am away; the spot dries up so I can no longer see it, and the odor remains. I had learned that I need what is called "black light" to find such old spots. I chose Simple Solution 11350 Spot Spotter Ultraviolet Urine Detector, a hand-held, battery-operated device that shines ultraviolet light. I don't know how it works, but when you shine this light in a dark room, urine stains become visible, they begin to glow with dull yellow.
The device works on carpet, tile, wooden floor, upholstery. It only works in the dark, making the stains visible. One minor complaint is that it requires 8 AA batteries, which I would say is quite a lot, and they are not included with the purchase. You can also use an adaptor cord, but it is not included either. I think the manufacturer should have provided either the batteries or the cord.