Was the beeping in a backyard coming from a missing woman's digital watch?

In the fall of 1996, a young mother named Mary Lassiter was looking for a house to rent in Arroyo Grande, California. She spotted an ad for a rental that a woman named Susan Flores had put in the local newspaper. The charming blue house with the picket fence on East Branch Street seemed perfect. In October 1996, Mary and her family moved in.

The Lassisters say soon after, strange postcards began showing up in their mailbox. They were meant for Susan Flores – asking her to cooperate with police and have her son come forward. That's how the Lassiters learned that their landlord's son, Paul, was the prime suspect in the disappearance of a young college student in nearby San Luis Obispo. 

Four months earlier, Kristin Smart, a freshman at California Polytechnic State University – better known as Cal Poly – had gone to a party on Friday, May 24 – the start of the Memorial Day weekend. People who were there have said she was intoxicated. Some even wondered if she might have been drugged. By the end of the evening she was found passed out on the lawn next door and needed help to get back to the dorm. A student named Paul Flores, Susan's son, volunteered to take her back. He was the last person seen with her.

More than two decades later, Chris Lambert, a podcaster who was investigating Kristin's disappearance, tracked down and interviewed Mary Lassiter. His podcast, "Your Own Backyard," has become a big hit – attracting millions of listeners eager to hear every detail surrounding this unsolved case. "48 Hours" contributor Jonathan Vigliotti takes viewers inside the popular podcast and haunting case in "The Disappearance of Kristin Smart." 

Over the years, Susan Flores's backyard has been the focus of widespread speculation because of stories like the ones Mary Lassiter told Chris Lambert. She told him that shortly after moving in, she was washing her car in the driveway and found an earring next to the left front tire. The earring was turned over to the San Luis Obispo Sheriff's Office, but investigators will never know if it belonged to Kristin, because it was misplaced. Lassiter told Lambert the earring she found matched the necklace Kristin was wearing on one of the "Missing Cal Poly Student" billboards.

But perhaps Mary Lassiter's most disturbing story was about being awakened very early in the  morning by a beeping sound coming from the backyard.  

As she told Chris Lambert, "In the master bedroom, I'm hearing this 'beep beep' every night at 4:20, or 4:20 in the morning." It sounded like a digital watch alarm, but Lassiter couldn't find a watch anywhere outside. The backyard was all concrete except for a couple of areas where planter boxes had been created by cutting out the concrete and filling them with soil. 

Chris Lambert told Jonathan Vigliotti, "It drove [Mary Lassiter] crazy and she went out there many times in the middle of the night to try to find it with sticks, sticking down into the planter boxes down in the soil."  She told Lambert that after several months, the beeping stopped. He believes that meant "the batteries went dead and that the object that was beeping hadn't been there very long."

When Lambert later told Kristin's mother, Denise, about those early morning beeping sounds in Susan Flores's backyard, he said her reaction was "shock … we just kind of stared at each other for a good long time … I think it took a while to settle in, that this could potentially be huge." And very chilling. Denise had told Lambert that her daughter had been working as a lifeguard at the university's rec center, and every morning Kristin's watch alarm went off to make sure she was at the pool by 5 a.m.

Over the years, law enforcement and the Smarts' lawyers have searched Susan Flores's backyard several times, but nothing has ever been found. Investigators could have excavated the yard, but their decision not to was a crushing blow for the Smart family. 

Kristin's parents have spent more than two decades hoping for any clue that would lead them to their beloved daughter. Even when law enforcement let them down, they refused to give up.  Over the years, while Stan Smart hiked every trail around California's Central Coast in hopes of finding Kristin's remains, Denise relentlessly pushed law enforcement to do more. The Smarts' younger daughter, Lindsey, described her parents to Vigliotti: "I think they're like carrying boulders on their backs."

In 2011, the Smarts hoped those boulders would become a little lighter when Ian Parkinson became the new sheriff of San Luis Obispo County. He promised the Smarts that solving Kristin's case would be a priority. He hired a detective to work full time on the case, and over the past nine years deputies have carried out searches, conducted new interviews, and gathered additional evidence. After all these years, Sheriff Parkinson says that Paul Flores continues to be their number one suspect.

Flores has never been charged in this case. He has never talked. His parents have always denied knowing anything about Kristin's disappearance and insist their son was not involved. Podcaster Chris Lambert says that after 24 years, "I think it's time [for Flores] to talk."  "48 Hours" agreed. Jonathan Vigliotti found Flores outside his home in Southern California – and asked him a number of questions about Kristin's disappearance.