Theo Hayez disappeared two weeks ago. His final WhatsApp messages could help solve his case.

As Theo Hayez walked back to his Byron Bay hostel from the Cheeky Monkey’s nightclub on May 31, he was on his phone.

The 18-year-old Belgian backpacker messaged a friend back home in Belgium just after midnight, but the contents of that message remain a mystery, as does Theo’s location.

He never made it back to his hostel and he was reported missing on June 6 when he failed to check out, leaving his belongings and passport still in his room.

Nearly three weeks on, there has been no sign of Theo. NSW Police have admitted they are “baffled” by his disappearance.

Watch: Fears grow for missing backpacker. Post continues after video.

His father Laurent Hayez arrived in the country last week and on Monday appeared at Tweed Heads police station to beg the public for help, saying his son was “in grave danger”.

“I promised Theo’s little brother Lucas that I would bring his brother home. Please, help me keep my promise to him,” Laurent said through his tears.

Theo’s WhatsApp messages.

Laurent asked encrpyted service WhatsApp for help in obtaining his son’s messages, which he believes may hold the key to finding him.

In a statement, WhatsApp said it was working with law enforcement but it was limited in what data it can access from Theo’s account.

“WhatsApp cares deeply about the safety of our users and our hearts go out to Theo Hayez and his family,” a spokesman told AAP on Monday.

“We understand the important work being carried out by law enforcement and are assisting them in accordance with applicable law and our terms of service.”

WhatsApp uses end-to-end encrpytion, a system of communication where only the communicating parties can read the message. Even if WhatsApp accessed Theo’s account, they could not have the encryption key needed to read his messages.

This key is on Theo’s phone and the only way to access it is by having possession of that device.

theo hayez missing
Image: Facebook.

Therefore, WhatsApp is only able to access and disclose information that can include a user's name, last seen date, IP address and basic information but not the content of messages.

Apple's iMessage and FaceTime also use end-to-end encryption (though this is only when both the sending and receiving Apple device have iMessage turned on - if your texts are green, they're not encrypted).

The search continues.

Dozens of people, including State Emergency Service volunteers and members of the public, on Tuesday returned to the area around Cape Byron Lighthouse where the last 'ping' from Theo's phone was recorded on June 1.

He had been preparing to return home after eight months travelling around Australia.

While homicide detectives are helping the investigation, along with Belgian authorities, Superintendent David Roptell said he held out hope that Theo was alive.

"We're just not clear at this point," he said. "It appears to be out of character. He was intending to go home. It's baffling as to what's occurred, but we're not ruling anything out."

Laurent Hayez thanked police, volunteers and the community for their help in looking for his son, as well as those who raised money to support the search.

More than $65,000 has been raised via a GoFundMe page for the Hayez family, part of which was used to help Laurent Hayez fly to Australia. He has vowed to stay until his son is found.

He said any crowdsourced funds not used to find Theo will be donated to a charity in Australia and Belgium dedicated to finding missing people.

Supt Roptell said it was "heartbreaking" that investigators did not have any fresh leads to share with Theo's family but promised not to leave any stone unturned.

-With AAP.

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