A mobile phone belonging to a backpacker who vanished in Byron Bay nearly three years ago was last tracked climbing a steep headland before it mysteriously stopped transmitting.
An inquest into the disappearance of Belgian Theo Hayez resumed this week, as police announced a $500,000 reward for information in his case.
The 18-year-old was last seen at about 11pm on May 31, 2019, when he was ejected from the Cheeky Monkeys bar in Byron Bay.
Police were alerted six days later, when he failed to return to his hostel and could not be found or contacted.
On Thursday the inquest heard evidence from a telecommunications expert whose analysis of Theo's mobile phone data places the teen on a headland below the town's famous lighthouse around midnight the day he disappeared.
Investigators have already aired evidence, gleaned from Google location data, showing Theo had spent seven minutes at a sporting field, before charting a route through the Arakwal National Park to the beach below the headland.
The phone data studied by Professor Aruna Seneviratne closely correlates with the Google data, but continues for an hour after the latter cut out.
It shows Theo's mobile connecting to two telephone towers several times between midnight and 1.02am on June 1, giving a snapshot of its location at those points in time.
The data is accurate with an error margin of 78 metres, Prof Seneviratne said.
He told the inquest he is "highly confident" it showed the phone moving away from the telephone tower and up the headland towards the lighthouse.
A police officer completed the climb from the beach, up the headland and to the lighthouse with a phone similar to Theo's to provide another point of reference.
That phone connected to the same towers as Theo's had, and the location data was very similar to that of Theo's phone.
However, the exercise also enabled Prof Seneviratne to conclude "with a high degree of confidence" that Theo's phone never reached the lighthouse, he said.
The phone then stops transmitting data after 1.02am, before coming back online between 6.17am and 1.47pm, but Prof Seneviratne says the data from that period is weak and not as reliable.
Prof Seneviratne said he could not determine why the phone stopped transmitting for five hours.
The data does indicate, however, that Theo's phone did not end up in the ocean.
"It cannot be in a blue area that represents water simply because a phone will not survive (and transmit) in salty water," he said.
The current police theory is that Theo clambered up the beachside cliffs, dropped his phone, then fell and was swept out to sea, something his family says goes against the teen's sensible, risk-averse nature.
Another search and rescue operator who searched the cliffs in the area with a drone and by abseiling down some told the inquest on Wednesday they were incredibly steep and crumbly.
"I've been off many, many, many cliffs and that one made me nervous," Senior Constable John Stirling said.
