Cats can hear things that humans cannot hear.
These little creatures also have much better hearing, not only compared to humans, but also to other mammals. For example, the hearing range of a dog is between 67 Hz and 45 kHz, while a cat is between 48 Hz and 85 kHz. This helps cats to detect even the slightest noise, such as stirring grass or gurgling water. So if they are looking at your wall, there may be a problem with the pipes or maybe insects have settled there.