Spraying small amounts of urine onto vertical surfaces is perfectly normal behavior for your cat, but can be a real nuisance to their humans (especially if the drapes or furniture happen to be the target). Cats define their territory, compete for dominance or announce their sexual readiness by spraying urine, a practice that is particularly common with unneutered males and in multiple cat households.
It can be difficult to narrow the cause of urine spraying, especially if there is more than one cat in your home. Try isolating the cats from each other to determine which cat is spraying urine. Or, with the guidance of your veterinarian, administer a dye test to establish which urine sprays belong to an individual cat. Your cat will need to swallow a fluorescein pill that will make his urine visible under black light. (You can use a portable black light wand to see where the urine is located.)
Spaying or neutering a cat can help block the urge to spray urine. In addition, limiting access to windows where an indoor cat may see -- and compete with -- an outdoor cat, can prevent your cat from feeling the need to mark his territory. Try providing some private space for your cat, such as a cat condo or high shelf from which he can watch his kingdom. Or use a calming spray like Feliway that mimics cat pheromones. This is a good way to reduce stress and the urge to assert dominance with urine spray [source: Fenichel].