The internet transparency entity WikiLeaks tweeted on Sunday about Arjen Kamphuis' "strange disappearance," saying he has been missing since August 20, when he left his hotel in the northern Norwegian town of Bodo.
WikiLeaks said that Kamphuis, an associate of founder Julian Assange, had a ticket for a flight departing on August 22 from Trondheim, which is over 700 kilometers (435 miles) south of Bodo.
"The train between the two takes (approximately) 10 hours," suggesting he disappeared either in Bodo, Trondheim or on the train, WikiLeaks said.
A website set up to gather information on the missing person says: "He is 47 years old, 1.78 meters tall and has a normal posture. He was usually dressed in black and carrying his black backpack. He is an avid hiker."
There have reportedly been two unconfirmed possible sightings, one in Alesund, Norway, and the other in Ribe, Denmark.
Norwegian police on the case
Norwegian police started looking into the disappearance on Sunday.
"We have started an investigation," police spokesman Tommy Bech told the French news agency AFP, adding that so far they had "no clue" where the Dutch citizen was.
The police "would not speculate about what may have happened to him," Bech said.
Norwegian police can't legally access his cellphone movement data until Kamphuis is officially reported missing in the Netherlands, according to the Norwegian Verdens Gang tabloid newspaper.
The Dutch Ministry of Foreign Affairs said it was aware of his disappearance after Kamphuis' friend and privacy activist Ancilla van de Leest tweeted about his disappearance.