Does the cat have special abilities?

From the text, it doesn't appear as if the cat has any special abilities. Instead, the cat is more of a symbol. It can be argued that the cat is a symbol of the people's paranoia. At the same time, the animal may also symbolize the futility of trusting in any one method as a means of protection. Perhaps, the cat also highlights the need for wisdom on the part of the townsfolk in Gordimer's story.
In the story, the little boy finds himself caught in the razor-sharp wires at the top of the wall. His flesh is torn as he struggles to free himself, and he dies from his injuries. While the boy is struggling to disentangle himself from the barbed wires, the cat trips the alarm. The tripped alarm sets off a wailing that corresponds with the screams of the boy. The irony is that neither the alarm nor the barbed wires prove useful for the purposes intended. Instead of keeping out violent criminals, the alarm accompanies the screams of a boy in the throes of a painful death. Interestingly, the cat is not injured by the barbed wire, despite the wife's concern.
So, the cat symbolizes a few things: the need for wisdom, the people's paranoia, and the futility of a knee-jerk reaction to danger.