The New York Times has a brief article on why we have a tendency to see faces in chaotic or almost random visual scenes.
Although it is controversial as to whether it is specifically dedicated to recognising faces, an area of the brain known as the fusiform gyrus is certainly heavily involved in perceiving faces.
The fact that this area is so specialised for faces might lead us to detect faces even when they are only suggested by a few dots, the position of clouds or the markings on just about anything.
“The information faces convey is so rich ‚Äî not just regarding another person’s identity, but also their mental state, health and other factors,” he said. “It’s extremely beneficial for the brain to become good at the task of face recognition and not to be very strict in its inclusion criteria. The cost of missing a face is higher than the cost of declaring a nonface to be a face.”
There’s a great web page with pictures of ‘cloud faces‘ if you want to see how spectacular some of these effects can be.
Link to NYT article ‘Faces, faces everywhere’.