Kristin Smart vanished in 1996. Then Mary and Joe heard a mysterious alarm in their backyard.

In June 1996, Mary and Joe Lassiter moved into what they imagined was just an average rented house on East Branch Street in Arroyo Grande, California.

But it didn’t take long for the Lassiters to realise that perhaps their new home was not so average after all.

For starters, there was a fresh layer of concrete over the backyard, which had been laid just weeks beforehand.

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Then there was the red splatter in the bathroom. Mary, a nurse, and Joe, a hospital worker, instantly recognised it as dried blood.

In October, as Mary searched the bizarrely concreted backyard for a spot to plant flowers, she discovered a lone, blood-smeared earring.

But the most obvious sign that something weird was going on happened in the middle of the night.

Every night.

At exactly 4.20am each morning, the couple would wake to the sound of an alarm. It sounded like a watch alarm – beep, beep, beep – every day for weeks, perhaps even a month.

With the headboard of their bed against the wall that backed immediately onto the backyard’s freshly laid concrete, Joe and Mary became far too well aware of the suspicious history of the house on East Branch Street.

A disappearance.

A month before Joe and Mary had moved into the home, 19-year-old Kristin Smart had vanished.

She was at the end of her freshman year at California Polytechnic State University, located in San Luis Obispo, about 28 kilometres north of Arroyo Grande.

May 24 marked the Memorial Day holiday weekend, and Kristin had attended a birthday party for a classmate on campus.

At approximately 2am on the morning of the 25th, fellow students Cheryl Anderson and Tim Davis had just left the party when they found Kristin passed out on a neighbour's lawn.

They helped her up and decided to walk her back to her dorm room. Then a third student, Paul Flores, offered to join them.

Tim, who lived off campus and had driven to the party, left the group first. Then Cheryl was told by Paul he could take Kristin the rest of the way because he lived closer.

According to Paul, he walked Kristin as far as his dormitory and then allowed her to walk back to her dorm alone.

That was the last time anyone saw Kristin Smart.

She was declared presumed dead in 2002, but her body has never been found.

Paul Flores.

Hours after Kristin was last seen, Paul Flores was spotted with a rubbish can loaded onto a utility cart.

The next morning, Paul, who was also 19, woke up with a black eye.

The Daily Beast reported when questioned, he had multiple stories: First, he said he'd got it during a basketball game, but when his friend said he'd turned up to the game with it already, Paul said he'd got it working on his truck.

Later, he told a friend he had just woken up with it and couldn't recall how he got it.

On May 28, a missing person's report was filed for Kristin. In it, her roommate said that none of her belongings - her ID, toiletries and clothing - had been touched in their dorm.

On June 29, more than a month after Kristin disappeared, local authorities led cadaver dogs to the dorm Paul shared with his roommate, Derrick Tse.

Each dog alerted on a corner of a bed mattress on the left side of the room: Paul's side.

Police removed the mattress as evidence and sent the dogs back in separate operations. Even with it gone, the dogs alerted to the left side of the room.

Police were confident this meant either a dead body was in the room, or someone who accessed that room had touched a dead body.

Derrick told police Paul had told him "Yes, I killed her and brought her to my mum's and she is still there".

The concrete slab.

The house Joe and Mary Lassiter moved into in June 1996 was owned by Susan Flores, the mother of Paul. She had lived there up until very recently, including during the time Kristin Smart disappeared.

Before the Lassiters moved in, a then-21-year-old food prep cook who lived across the street saw something that still haunted him "like it was yesterday".

As he stood at his kitchen sink, the young man watched Paul and another unknown male take turns shovelling and wheelbarrowing to create a four-feet-deep hole in Paul's mother's backyard.

In the middle of the construction effort the man, who remained anonymous, told The Daily Beast he saw both men grab a rolled-up rug and lug it into the hole.

"I’ll tell you as a 15-year floor layer I can take a whole room of carpet, roll it up like a burrito - we call it cockroaching - you throw it on your back and walk it upstairs," he said.

"Two people were needed to move this. So it was heavy. And that’s the thing I’m tripping on."

He said the two young men backfilled the hole and poured concrete on top to form a slab.

A week later, Paul's face was featured on the local news in regards to Kristin's disappearance and the neighbour instantly recognised him as one of the diggers.

By the time the alarm sound Mary and Joe heard each day stopped, they were more than aware of the suspicions surrounding their landlord Susan's son, and they began to put together a puzzle.

They recalled how Paul's father Ruben Flores had been desperate to retrieve an old metal rubbish bin in the backyard after they moved in. It was after he removed it that Mary found the one blood-smeared earring.

Silver and turquoise, she believed it to be an exact match to the necklace Kristin wore in the missing person's billboard she had seen. Submitted as evidence, the earring was photographed by detectives but shortly after, police told Kristin's family they had lost both the photo and the earring.

As they had been woken by the alarm at 4.20am each morning, Joe and Mary had been unaware that Kristin woke early each day to work a 5am shift as a lifeguard before her college classes.

But despite all clues pointing towards Paul Flores, he has never been charged.

Three months into their lease of the East Branch Street house, after the Lassiter's began sharing their discoveries with police and media, Susan evicted them.

Search warrants and a civil suit.

In July 1996, authorities searched the home of Ruben Flores.

In March 1997, Susan's Arroyo Grande property was searched using ground-penetrating radar technology to look underground. The results were inconclusive.

In November that year, Kristin's family filed a wrongful-death suit against Paul that remains unsolved.

During a deposition, Paul only confirmed his name and birth date. In response to every other question, Paul invoked his Fifth Amendment right not to answer, even in response to innocuous yes/no questions and questions about the names of his relatives.

Sexual assault claims.

In 2016, a Daily Beast investigation reported four women alleged they had been sexually assaulted by Paul Flores.

One woman, speaking under the pseudonym Jane, told the publication she was roofied, raped and discarded by Paul and a nameless friend when she was 15 years old in 1994.

After going to the police, Paul and his friend claimed the teen was drunk and it was consensual. Though she fought it, Paul was never charged.

Three other women said they were assaulted by Paul after Kristin's 1996 disappearance.

New breakthroughs.

Beginning in September 2019, a seven-part podcast by American journalist Chris Lambert renewed interest in Kristin Smart's disappearance.

The Your Own Backyard podcast spurred police on to seize "specific items of evidence" from four locations related to the Flores family and Paul was briefly detained. In late January, they seized two trucks owned by him.

Kristin's mother Denise Smart told The Stockton Record she had been contacted by a retired FBI agent who told her to be ready for a development that could help to bring her family closure, nearly 24 years after Kristin went missing.

"Be ready. This is really going to be something you don’t expect. We want to give you the support you need," the former agent reportedly told her.