About 9.4 million U.S. homes, or 7.4% of the total, are equipped with Wi-Fi enabled cameras and mics, says Brad Russell, research director for Parks Associates, a consumer technology research firm. As many as 11 million or so have similar but more limited set-ups trained on the doorstep or outside the house, or embedded in a light fixture, Russell says. That means up to 13% of homes have at least one Wi-Fi camera and mic. The cameras often are visible but can be hidden in stuffed animals, like a “nanny cam,” or concealed in bookshelves. This Web-enabled do-it-yourself home surveillance market didn’t even exist five years ago, Russell says.

From the article "Buying a home? Sellers may use cameras, microphones to spy on house hunters" by Paul Davidson.