Maybe they’re not just quirky habits.

In the wake of alleged Gilgo Beach killer Rex Heuermann’s arrest, a forensic psychiatrist has broken down a series of potential behavioral red flags that appear consistent with the mannerisms of serial murderers.

Dr. Carole Lieberman, who has served as an expert witness at murder trials, warns that a series of subtleties — like isolation, a hot and cold temperament and even living in an area where famous crimes occurred — may all tie into the psyche of a psycho.

Her commentary comes at a time when many in Heuermann’s home nabe of Massapequa Park, who have all commuted with and lived near him for almost 15 years, are curious as to what telltale signs they missed as a suspected killer was literally under their noses on Long Island.

Don’t blame naive neighbors for this one, though. Lieberman will be the first to tell you that spotting the bad apples is anything but a perfect science — hence why so many stay on the lam or are never even caught.

“They’d have to be doing something really frightening during a one-time encounter to make you feel justified in contacting 911,” she told The Post, explaining why serial killers can hide so well in plain sight.

Instead, it’s consistent observations of unusual behaviors over time that would best indicate if a person may be living a murderous double life.

These are the collective behaviors Lieberman says to keep an eye out for.

Just having an unkempt house can be a warning sign, according to Lieberman.

Such was the case with Heuermann, an architect who lived in an underwhelming, red-painted home surrounded by half-million-dollar properties, the psychologist said.

“His house sticks out like a sore thumb. It’s foreboding that people don’t send their children there on Halloween,” she added of parents’ natural instinct to keep away from his home.

A house being dimly lit and having a tool shed are other creepy characteristics to watch out for.

On top of all that, a homeowner who keeps odd hours and is digging, doing yardwork or gardening at night — as neighbors reported about Heuermann — is also a major concern.

Lieberman advises that neighbors who spot sketchy behavior but are not ready to call 911 should first Google the person to better understand their background.

A “hair-trigger” temper that switches between excessive bursts of aggression to a cold and emotionless persona is often associated with murderers, according to Lieberman.

“Serial killers often do have hair-trigger tempers because, for one thing, they’re holding in so much. It’s keeping a secret for years and always being on guard,” she said.

“They are typically controlling. If something doesn’t go their way, then that rage is expressed at the drop of a hat. But deep down for the long term, they are superficial or ice cold.”

In the case of Heuermann, clients had described him as being “belligerent” with contractors while on the job.

Also, killers often don’t keep close friends and usually have a very quiet family life or marriage, Lieberman said, adding that they don’t volunteer much about their lives to people around them.

“They don’t want to give people too much information about them. They don’t want people putting pieces together. They’re afraid of being detected,” she added. “They’re always on their guard and they have to keep enough of their story hidden so that people won’t start asking more questions.”

A person’s environment can “come into play” as inspiration to become a killer and is something locals should be mindful of, said Lieberman.

“Not necessarily where they’re thinking [of killing], but maybe [the idea that] I’ll use the same techniques or maybe I’ll do it better. I won’t get caught.'”

While Heuermann’s part of the South Shore might appear to be a typical slice of suburban harmony, it has a notoriously checkered past.

In 1974, “Amityville Horror” killer Ronald DeFeo Jr. murdered his family in cold blood 10 minutes away from Heuermann’s home.

Years later, in 1992, “Long Island Lolita” Amy Fisher shot Mary Jo Buttafuoco, the wife of her affair partner, Joey Buttafuoco, in Massapequa.

The town’s nature preserve has also made headlines as bodies have been recovered in recent years related to major gang slayings.

Thoughts of outdoing rival killers and “the idea of fame” that comes with being a nicknamed iconic killer may also drive a person to darkness, Lieberman explained.

After being booked, Heuermann’s sole question to law enforcement was: “Is it in the news?”

Albeit the most challenging way to spot a mass murderer, Lieberman says that some public tendencies shouldn’t be easily ignored.

If someone seems oddly “transfixed” by an advertisement — especially of an attractive woman — to the point that “they’ve lost touch with reality,” Lieberman said it may be a sign that they’re spacing out and thinking of victims.

Another big sign is how people may react if you bump into them on a crowded street.

“If somebody passed too close to them and was touching them … like a light sort of erotic touch accidentally, that could get the killer’s attention,” she said.

“But if it’s a jostling touch, they would not like this. This could be one of the occasions for their hair-trigger temper. They don’t want you getting too close.”

Body language of people around a shady figure may also be a foreboding sign, too, especially a creepy man around a woman.

“If they’re sitting on a bench, she might have her legs crossed and her arms crossed with a protective of stance,” Lieberman said. “She might not look at him directly [and show] indifference to him.”