There's a real-life story behind why we have Amber Alerts, and it begins with a little girl in 1996.

Peta Hodgson, a 15-month-old girl from Sydney’s west, has been missing since Tuesday afternoon.

She was last seen at home with her mother, 29-year-old Sarah Hodgson, at around 2pm, but the pair are understood to have left the property, and haven’t been seen since.

Police have issued an Amber Alert as they search for Peta, citing ‘extreme concerns’ for her safety. They are requesting the public be on alert, and report any sightings.

Amber Alerts are serious, and vitally important. Most of us know that they’re designed to attract the public’s attention and mobilise people to help in a dangerous situation. But what’s less known is how and why they started.

The case of Amber Hagerman inspired the creation of the alert system.

The story begins on January 13, 1994, when nine-year-old Amber Hagerman, who was riding her bike in a vacant supermarket parking lot, was abducted.

Detective Ben Lopez of the Arlington Texas Police Department still remembers that day, as he was a patrol officer in the area at the time. Lopez was put on to a special task force to find Amber, with their only clue being a dark-coloured truck sighted at the scene.

“For those first few days, we spent all of our extra time looking,” Detective Lopez told Dateline NBC.

“It was like if you weren’t on another call, you were actively looking for her. We were looking everywhere in the city.”

The brazen kidnapping gripped the nation as Amber’s parents made desperate pleas for her safe return through the media. But tragically, on January 17, Amber’s naked body was found in a creek only kilometres from where was was taken. Her throat had been slit.

Families across the country mourned for the Hagerman family, including Diane Simone, a massage therapist and mother from Dallas. Diane called a local radio station, asking if local broadcasters could work with law enforcement to get information out immediately following a child abduction.

It was a revolutionary idea at the time, and one that would go on to be constructive in locating more than 800 missing American children almost immediately after their disappearance.

The term Amber Alert actually stands for AMBER: America’s Missing Broadcast Emergency Response, with the acronym intentionally honouring the little girl who inspired its creation.

How does an Amber Alert work?

In its two decades of operation, the system of notifications has evolved from mobile phone alerts to social media posts, with Facebook announcing a partnership with the National Centre for Missing and Exploited Children in December 2015.

Queensland has used the Amber Alert system since 2015. The Queensland Police Service issued 12 alerts in 2016 for matters relating to high-risk missing children.

All Australian states joined the Amber Alert Facebook initiative in June 2017.

The system works with state and territory police to push urgent child abduction alerts onto a Facebook user's news feed if they are in the same area as the missing child.

Strict criteria means only urgent cases where a child is at risk of death or serious injury and there is information, such as a registration plate number, that can help the public find the child, is available to be used in the alert.

#Breaking - Amber Alert Update - 12yo boy has been located safe by police in NSW. Final Amber Alert media release will be issued soon. Police still looking for the 2013 Jeep Compass 760 TNH #AmberAlert #Mudgeeraba pic.twitter.com/DtCmNBeTKo

— Queensland Police (@QldPolice) May 12, 2018

The alerts are usually targeted to the public in the relevant locations, and are extremely time-sensitive.

"Time is often of the essence," Detective Superintendent Cheryl Scanlon, from Queensland Police Service's child abuse and sexual crime group, recently told ABC Radio Brisbane.

"What can happen is these things ramp up very quickly and it can be a matter of minutes actioning these things once a decision is made to go Amber.

"For the time-critical ones, it goes to a very senior level and it doesn't matter what time of day or night, we take calls on these."

Amber's legacy.

More than 20 years since the innocent child's life was taken from her, Amber's kidnapper has never been found.

Her mother, Donna Williams, told Yahoo News on the 20th anniversary that it has naturally been a challenge for her to stay positive.

“There’s a part of me that wants to be happy,” she said. “But then there’s a part of me that says I can’t, that I shouldn't be happy.”

Williams also spoke of writing an open letter to the killer, in a hope that he/she makes contact, so there can be some sort of closure. But Williams was never able to bring herself to send the letter, because the thought of such contact was too overwhelming.

Hopefully, the Amber Alert system will continue to prevent other families from experiences such heartache and loss, and keep more children safe.