Urinating in unusual places can often be a sign of stress. It takes very little to stress out a cat, has something recently changed? The hours you work? The placement of the furniture? A new human or animal in the house? My cat had abnormal urination patterns when his companion was separated from him, it wasn't that he had un-toilet trained himself, it was that he was suffering from separation anxiety.
As others have said, make sure the litter trays are always very clean. Remove feces and urinated litter. If there is too much urinated litter in my cat's litter box, he won't use it, the smell is too strong for him, so now I remove all urine soaked litter whenever I notice it. When the time comes to completely change the litter (usually twice a week) scrub the litter box completely.
Also mentioned above is that abnormal urination patterns can be a sign of urinary tract infections, consider taking your cat to the vet, as it will become extremely painful, and very expensive to treat if allowed to develop. My brother's cat just came back from having a UTI treated, and it set them back $3,000.