Hearing prospective buyers bad-mouthing their house through a nanny cam was devastating for Kristen Foote's clients, who took great pride in the home they were selling.

"Things like the home wasn't kept up; it was in a bad location; the kids' rooms were badly decorated," recalled Foote, a Realtor with Berkshire Hathaway, Fox & Roach on Rittenhouse Square. "It's hard for sellers when they hear negative things."

With the evolution of the smart home, it's not surprising that we are being watched or listened to, even in private homes. Whether ensuring that nothing is stolen or hoping to get some real-time feedback, anxious sellers may rely on nanny cams, baby monitors, Nest cameras, laptops or doorbell cameras. Though recordings have certain advantages, experts warn that it is also risky, and in some cases illegal.

Although laws differ by state, in both Pennsylvania and New Jersey, homeowners can legally videotape people visiting their home without consent (unless someone is partially or fully nude).

"The real focus with videotaping someone is their expectation of privacy, and in an open house situation, there is generally no expectation of privacy," said Alan Nochumson, a real estate lawyer in Center City.

It is illegal in both states, however, to audio-record people without their knowledge and consent. Pennsylvania, a two-party consent state, requires that both parties involved in audio-taping must agree.

"Though N.J. is a one-party consent state, a homeowner utilizing a nanny cam can't provide such consent because only a person participating in the conversation being recorded can provide consent under N.J. law," said Kate A. Sherlock, an intellectual-property lawyer with Archer Law in Haddonfield.

While there is generally no expectation of privacy in an open house setting, it's always safer to provide notice, which might come in the MLS, through a posted sign, at an open house sign-in table, or in marketing materials saying the house will be under surveillance during showings.

The Pennsylvania Association of Realtors doesn't offer formal recommendations, leaving it up to the discretion of each agent, said Hank Lerner, director of law policy. "If devices are capable of recording audio, agents do need to warn people about it," he said. "Technically, there's no rule that says you can't record video."

Sometimes, recordings give sellers peace of mind, especially during an open house when people might be exploring a home unaccompanied. "We've certainly heard stories of jewelry and small electronics going missing, and people looking in the bathroom for prescription drugs," said Lerner, who always advises people not to leave valuables around.

But videos cannot legally include audio. Though odds are that a homeowner recording with a Ring doorbell wouldn't get caught, "if you haven't gotten consent, you run the risk of criminal prosecution," Lerner said.

After one open house, Realtor Mike McCann got a call from a client who had watched her nanny cam video and seen a woman rifling through her bedroom drawers. Not knowing who the woman was, McCann posted the video on Facebook and received 300 responses, including 10 positive IDs.

"I ended up tracking the person down," recalled McCann, from Berkshire Hathaway, Fox & Roach Realtors, in Center City. Though nothing had been stolen from McCann's client, other Realtors noted that the same woman had been coming to their open houses for months, and in some cases, things had been missing afterward. McCann sent the woman a note saying he had the video and "people will be looking out for you at open houses."

Yet McCann insisted that, in his experience, theft was an isolated incident, and he's rarely seen recording devices on the job — maybe in 10 percent of homes. It's not a conversation he has with his clients, either when showing a home for sale or looking to buy.

"If crime were a big issue, it would come up more, and we would probably take more action," he said.

Electronic monitoring is not only about security. Some home sellers hope to glean information that might help in a negotiation.

Andrew Frank, branch manager of Long & Foster in Center City, said the number of homes with recording devices increases along with the home's cost.

"It depends on the location and price range; almost 60 percent from the $500,000 and up range" have surveillance devices, he said. "And that's when you know they're there. When you can get a camera for $150 and run it through your phone, it's efficient. When someone comes in and out of your home, it texts you and emails you immediately, so your natural reaction is to look at it."

For the last year or two, Frank has mentioned to clients looking for a home that they might be on camera and to watch what they say while in the house. On one occasion, while Frank was at the alarm keypad, he heard the homeowner on a speaker giving him alarm reset instructions. "He was watching the whole time," Frank recalled.

Overheard comments can, in fact, affect negotiations. When Tammy McMullin's client spoke openly about how much he liked certain features in a home, his wife, noticing the camera, downplayed the positives.

"You don't want to influence the transaction, where the buyer isn't in the best position because something was overheard by the cameras," said McMullin, Realtor with Keller Williams in Center City. "Everything you talk to your agent about should be confidential."

The same holds true for a home inspection if the seller overhears a buyer discussing which problems would endanger a sale and which wouldn't. "Some things could sound a lot worse in person, but when they get the report, it might not be that big of a deal," she said. "Or vice versa."

Overheard comments have even squashed sales. Jeff Senges, a Realtor with Berkshire Hathaway, Fox & Roach in Voorhees, recalled hearing at a sales meeting about a seller who was offended when prospective buyers refused to take off their shoes during a showing.

"The buyer ended up liking the house and put in an offer, and the seller refused to sell it to them," said Senges, who wasn't personally involved in the negotiation.

Talking about potential surveillance is tricky and not a conversation he has often with clients, Senges said. If he happens to see a camera in a home, he will point it out to the buyer, but "that could be insulting to a client," he said. "It's like insinuating they are going to act inappropriately."