On 20 August, Arjen Kamphuis, a leading Dutch cybersecurity expert, checked out of his hotel in Bodø, northern Norway. He had told friends that he planned to take the train to Trondheim, 10 hours away.

He never boarded the train. Nor, two days later at the supposed end of his holiday, did he catch his return flight to Amsterdam. An intensive search by Norwegian police, and two Dutch investigators dispatched to help them has failed to locate him.

A kayak believed to belong to Kamphuis, who advised governments, corporations, journalists and activists on information security, was pulled from the sea about 50km from Bodø on Thursday, police said, the day after an amateur fisherman found some of his belongings – reportedly including an ID card – floating in the water.

But mobile phone records show that 10 days after the Dutchman was seen leaving his hotel, both his work and personal mobile phones were briefly switched on – with German SIM cards inserted – more than 1,700km from the small northern town, at Vikeså near Stavanger.

Police said on Thursday they were “holding all possibilities open in respect to what might have happened” to Kamphuis and pursuing three distinct lines of inquiry: a “voluntary disappearance” including a possible suicide; an accident; or foul play.

Foldable kayak pulled from the sea by police in connection with the Arjen Kamphuis case the day after an amateur fisherman found some of his belongings – reportedly including an ID card – floating in the water. Photograph: Norwegian Police Handout/EPA

But if investigators have no clear idea of what happened, the internet has plenty - mainly because on 1 September the whistle-blowing website Wikileaks tweeted to its 5.4 million followers: “@JulianAssange associate and author of ‘Information Security for Journalists’ @ArjenKamphuis has disappeared.”

The anti-secrecy organisation has since published several updates and comments on the investigation, describing Kamphuis’s disappearance as “strange” and highlighting the fact that his phone was activated a long way from where his belongings were found.

The site’s followers have suggested variously that Kamphuis, an outspoken campaigner for online privacy rights, was kidnapped or even murdered by the US, British or Russian secret services, or perhaps by Islamists.

Alternatively, they speculated, he may have gone undercover on a secret mission for Assange, the group’s founder, who has been living in the Ecuadorean embassy in London since June 2012.

Police have denied any connection between Kamphuis’s disappearance and the fact that Bodø houses a Norwegian military airbase, and that buried deep inside a nearby mountain is a major part of the Scandinavian country’s secret cyber defence operations.

With an internet campaign using the hashtag #FindArjen to locate him, unconfirmed sightings of Kamphuis have also been reported in Sweden, Denmark, Germany and several parts of Norway.

The Dutchman’s friends have accused Wikileaksof using his disappearance for its own benefit, and are upset that some media reports have unquestioningly repeated the organisation’s claim was an Assange “associate”.

.@JulianAssange associate and author of "Information Security for Journalists" @ArjenKamphuis has disappeared according to friends (@ncilla) and colleagues. Last seen in Bodø, #Norway, 11 days ago on August 20. pic.twitter.com/dV75NGKpgI — WikiLeaks (@wikileaks) August 31, 2018

“I hate how overblown Arjen’s Wikileaks connection is becoming,” said one, Ancilla van der Leest. “He advised many parties and journalistic organisations on infosecurity. In this light he was also a ‘Reuters associate’.”

Dutch media have reported that the Dutchman helped some Wikileaks members with advice on avoiding cybersnooping and government surveillance, the subject of a book he co-wrote with Silkie Carlo, but it is unclear how often or regularly he worked with the site.

There are also unconfirmed reports that he visited Assange in Ecuador’s London embassy, where the Australian is living because he fears extradition to the US if he leaves the building.

Carlo, who is also the director of Big Brother Watch, tweeted: “WikiLeaks might want to make this sound like it’s about them, but it is not ... It makes me, and others, feel sick to my stomach to see Arjen being missing/out of contact reported like a WikiLeaks murder mystery.”

Carlo said Kamphuis had “gone off grid before. It can be good for the soul. I was worried before, and it is worrying now. But I have strong faith he will come back into the welcoming arms of his friends when he is ready, in his own time”.

Van der Leest told Dutch radio that while Kamphuis “certainly didn’t come across as someone who was planning to be away for a long time. He had lots of appointments”.

Dutch police have said Kamphuis bought a foldable kayak before leaving the Netherlands, telling the salesman he was going canoeing in the fjords.

Other friends said Kamphuis was no daredevil. “If he’s heading into dangerous territory, he always seeks out company,” Helma de Boer told NRC Handelsblad. “And he always has good equipment. His motto is: ‘better safe than sorry’.”