It’s the mystery that has captivated the world since May 3, 2007: as her parents dined at a restaurant just 50 metres away from their holiday apartment in the coastal town of Praia da Luz, Portugal, three-year-old Madeleine McCann disappeared from her bed.

She was last seen by her dad Gerry, who returned to the apartment at approximately 9:05pm to check on Maddie and her two younger twin siblings as they slept.

When Maddie's mother, Kate, checked the apartment almost an hour later, Maddie was gone.

After 10 years and almost $20 million worth of investment into the investigation, and there is still no clear idea of who took Maddie, where she is and if she is still alive.

Now, an investigation by Channel Seven's Sunday Night has revealed that Scotland Yard is desperate to pursue one last line of enquiry they believe might help finally solve the case and find Maddie.

"There is an employee, somebody who worked within the Ocean Villa complex who has some information or some knowledge that may be of assistance," former Scotland Yard police officer Colin Sutton to Sunday Night reporter Rahni Sadler.

It is unknown whether the employee in question is male or female, or what their role was at the holiday resort, but detectives believe finding this 'mystery' witness could bring them closer than ever to finding Maddie.

In startling comparison, ex-Portugese police chief Goncalo Amaral, who has been battling the McCanns in court after he wrote a book claiming they were responsible for Maddie's death, told Sunday Night he believed the British secret service is involved in the "cover up".

Although he wouldn't state exactly how he thinks they were involved, Amaral believes "for sure" MI5 had some sort of involvement in either helping to hide Maddie's body, or to cover up the McCanns' alleged crime.

Despite the public scrutiny over their involvement in Madeleine's disappearance, and having faced a decade of failed leads and investigations, the McCanns say they will never stop looking for their little girl, who would be celebrating her 14th birthday on May 12.

"I don’t believe we could ever reach a point where we just think, 'Oh well, we've done everything now'," Kate told Sunday Night.

"Madeleine's out there and she needs us to find her."

You can watch Sunday Night's full investigation here.