The search for Belgian teenager Theo Hayez, who went missing in northern New South Wales 11 days ago, has been narrowed down to two areas near the Byron Bay town centre through the use of phone records.

Theo Hayez, 18, vanished after leaving Cheeky Monkey’s bar about 11pm on Friday, 31 May. He has not been seen or heard from since.

Hayez’s belongings, including his passport, were left untouched at the Wake Up! Hostel in Byron Bay. Staff raised the alarm when he did not check out as planned.

He has not used his mobile phone or made any financial transactions since that time, NSW police said.

But Hayez’s mobile phone records have helped investigators narrow down the search area to east of the Byron Bay CBD and in and around bushland to the north-east.

“Our last recorded phone records indicate the phone was used in and around this area,” Detective Chief Inspector Matthew Kehoe said in Byron Bay on Monday.

“We haven’t located anything of significance thus far but our search continues ... It is very unusual behaviour from Theo. He is usually very regular in the contact with his family, hence we do have concerns for his welfare.”

Hayez was supposed to travel to Melbourne before returning to Belgium this week, according to a post on Facebook by a friend, Michael Dorkhom. Police said they had serious concerns for his welfare.

Kehoe said one inquiry police were pursuing was whether Hayez was with someone else when he disappeared.

“We believe that someone must have seen Theo after he left the licensed premises,” he said.

Alongside the efforts of NSW emergency services, locals and backpackers have also been searching for the missing teenager. Volunteers have met outside the bar where he was last seen since Saturday to scour the surrounding area.