Sometimes, it takes a little nudging for buildings to acknowledge their vulnerability. Mr. McLaughlin is installing a system at a property that was easily breached by a security team doing reconnaissance in advance of a visit by “a person of prominence within the government,” he said. “The agent got onto the roof of this particular building, then located the resident manager and said, ‘Your building is not very secure.’ ”

The board’s members acted swiftly, perhaps more concerned about their own safety than the safety of the bigwig who was planning a visit to the neighborhood.

Beyond their security uses, however, cameras can function as a kind of management tool.

A few years ago, a co-op on the Upper East Side upgraded its surveillance equipment “partly to monitor our night man,” said Phil Ginsberg, a longtime member of the board. “Everyone liked him, but he had a history of falling asleep. With the new system we could monitor him. He’s still with the building, but he’s on probation.”

Mr. Dolin says he has been getting more calls from condo boards intent on catching residents who are “hoteling” — offering their apartments for short-term rental — and from landlords bent on collaring tenants of rent-stabilized apartments who aren’t being truthful about their primary residence. “We bring in cameras and put them in the hallways,” Mr. Dolin said.

For his part, Mr. McLaughlin has been hired to get to the bottom of the matter when “someone brings his dog out to the stairwell to do its business and the building wants to find out who it is,” he said.

Meanwhile, a camera in a condo on Jane Street recorded a couple in the elevator who were “in flagrante delicto,” according to Aaron Shmulewitz, who heads the co-op and condo practice at the law firm Belkin Burden Wenig & Goldman. “They were renting from the unit owner,” he said. “Suffice it to say that the board pressured him not to renew their lease.”

Video evidence can sometimes also come in handy in legal matters. When a pedestrian fell in front of the co-op managed by Mr. McCool, and sued the building for $8 million, things looked bleak, said the board’s president, Dennis Paget. “But the surveillance camera on the perimeter of the building had recorded what happened,” he said, “and the judge threw the case out.”