This last week, a friend of mine posted a video on Facebook asking if anyone recognized this man in the video. Apparently, while they were out of town on vacation, a somewhat suspicious-looking man pulled into their driveway, got out of his pickup, and walked up to their outbuilding to check the door to see if it was unlocked. As he determined the door was locked, he turned toward the house, looking right into the camera, then got back into his pickup and left.

I’m beginning to see this type of thing on Facebook all the time. While watching these videos, it’s shocking to see the boldness with which some of these thieves will resort to. Many times they are stealing FedEx or Amazon Prime packages right off the front step. I’ve often thought this is the best form of advertising for these security companies, and I’m sure it helps to increase their sales.

The number of homeowners that have these security cameras is expanding very quickly. The trend has been fueled by the spread over the past five years of inexpensive Wi-Fi enabled cameras and mics that homeowners can buy and set up themselves for home security. Motion sensors notify them by text or email that a visitor is in front of their home, and they can then observe them on a computer, laptop or smartphone through the internet. Alternatively, they can view a recording later.

As a Realtor, I’ve noticed it’s actually becoming quite common to see a “Ring Doorbell” while showing homes to potential home buyers these days and a growing number of home sellers are using these security cameras and microphones to spy on potential buyers as they tour their houses or condos. They then may use what they hear or see as leverage in price negotiations. It’s become so commonplace that many Realtors today are advising their clients to be careful what they say inside the seller’s home. Before we walk in the door, I routinely tell our buyers to curb their enthusiasm while they’re in the house. I’ll say, “Pretend the seller is home” or “Don’t act too excited about this house while we’re inside. Just pretend somebody is listening.” Because you never know.

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.

In a survey conducted earlier this year by Harris Poll for NerdWallet, 15% of Americans who have ever sold a home said they’ve used surveillance cameras to monitor potential home buyers.Holden Lewis, a housing analyst for NerdWallet stated, “In a competitive housing market, everything is fair game.”