Several cats can traverse the same territory at different times marking the same areas over and over again. Marking appears to be time related. As the scent wears off, the cat comes back to mark it again. An indoor cat that sees another cat outside the patio door, may mark inside. Bringing the scent of another cat in on your shoes or clothing can have a similar effect.
Marking can be a sign of stress.
In multiple cat households, overcrowding can lead to urination problems. Cats are not pack animals and need their own space. Once an area is marked, it is important to clean it thoroughly or your cat will refresh his scent again and again. A cat that is frightened while in the litterbox could be too scared to use it. He could have been scared by a loud noise, a person or another pet coming upon him suddenly.