An NYPD sergeant checking on his elderly parents Friday made a horrifying discovery — they were dead in their Queens home, along with two others, from apparent carbon-monoxide poisoning, sources told The Post.

Robert Hugel, assigned to the NYPD’s Technical Assistance Response Unit, found the victims just after 3 p.m. at the house at 260th Street and 80th Avenue in Bellerose, the sources said.

Jerry Hugel, 83, was in the passenger seat of an idling car in the garage, which sources believe was the cause of the contamination. Marianne, 80, was found in a nearby basement kitchen.

The body of family friend Walter Vonthadden, 76, was found seated in the living room, and neighbor Gloria Greco, 70, was found dead on the home’s second floor, at the top of the stairs.

Neighbors were stunned. “It is very surreal,” said Barbara Stoja. “We knew them for over 35 years. They were very fun people; they were dancers.”

The German-immigrant couple led a Bavarian dance troupe called the Schlierachtaler Stamm Club. Their children — the Hugels had two sons and a daughter — were also active members of the dance club, including Robert, who served as one of the organization’s secretaries.

Dancing figurines often lined the front of their home, neighbors said.

“This is something you see on TV,” a tearful Stoja said. “It is so sad — we all know each other — to know he’s not ­going to be there anymore.”

Stoja described the ­elder Mr. ­Hugel as an avid gardener.

“He was just talking to my husband about planting tomatoes,” she said. “He and my husband compare gardens all the time.”

Another resident referred to the Hugels as the “nicest couple” in the neighborhood.

The Hugels were often seen around the neighborhood walking their golden retriever and another dog, neighbors said.

Authorities at the scene were unaware of any pets, but said any animals in the house would have died.

Additional reporting by Beckie Strum