Any sudden change in cat behaviour shows that something has disrupted its routine. The cat may be upset about a change in the household, a different kind of food or new pet, for example. The cat might also be interested in the corner for a reason that seems mysterious. Some cats like to hear the sound of their own voice echoing off the walls or in the bathtub. Other cats like to revisit places in the house where they once caught something, like a spider or insect. By observing your cat's behaviour closely, you may be able to understand what it is doing in the corner and why.
Outside the Litter Box
It is important to watch the cat to make sure it is not using the corner as a litter box. Cats will commonly use corners to urinate if they are upset about a change in their life. This is one way they can show how upset they are with the new situation. The litter box should be accessible and regularly cleaned, and if the household has a new cat, the others may refuse to share the same litter box. Urine is used as a calling card to other cats, and is a more distinct marker in the corner than in the open centre, which is another reason your cat may be in the corner.
If the cat is suddenly using the house as a litter box, or is unable to control its bladder, then it is likely a health issue and should go to the veterinarian. The same may be true if your cat seems uninterested in its normal activities and sits in the corner looking bored or inattentive. It is a good idea to find out the date of the cat's last check up and make sure the check up covered any problems relative to its age such as diabetes, kidney and heart problems. Older cats do become somewhat inactive, but a sudden change could point to an internal health issue.
The cat may be actively interested in the corner as well. If the cat is meowing or making noise in the corner, you may see if it is trying to get your attention, or simply sensing something else there. Cats sometimes like the sound of their own miaow. Elderly cats with poor hearing sometimes miaow into corners to amplify the sound. It is possible that the cat is simulating feline company or simple entertaining itself.
If the cat is sitting in the corner with its eyes open wide and ears upright in an alert position, it may be listening to another creature's movements. There may be spiders, ants or even moths crawling on the walls or floor in the corner. The cat could also be listening to creatures inside the walls, such as mice, bats or even squirrels, but this is much less likely because you would also be able to hear rodents yourself. Even if there are not any insects your cat may even remember finding an insect there before, and continue looking in the same spot later.
Cats and all other animals sense the world differently than humans. The corner may hold the cat's interest for another reason. Some cats are interested the shadows in the corners, cast by the moulding on the ceiling or the furniture. Others often stare into space or at the wall for some time, only to look away abruptly after 15 minutes. Cats also use their body scent to communicate, so if the cat is rubbing the corners of its mouth or the base of its tail in the corner, it is leaving its scent. In the wild this behaviour is a marker to show other animals it lives nearby. Domestic cats still follow their instincts from the past.