OSLO — In a remote Norwegian town north of the Arctic Circle, a cybersecurity expert with ties to WikiLeaks checked out of a hotel, dressed in khaki hiking gear and carrying heavy baggage.

That was on Aug. 20. No one has reported seeing him since.

The disappearance of Arjen Kamphuis, 46, has so far flummoxed a widening police investigation that has chased stray clues and false leads in Norway, Denmark, Germany, the Netherlands and Sweden. Norwegian police have released statements saying that they have no idea whether he was a victim of foul play, but that they “are open to all possibilities.”

The uncertainty, and Mr. Kamphuis’s links to WikiLeaks, the anti-secrecy organization that has run afoul of governments and other powerful interests, has the internet buzzing with conspiracy theories. Some suggest a kidnapping — or worse — involving Russia, the C.I.A., MI6, Islamists or the Clintons, while others ask if he intended to disappear, possibly on some secret assignment for WikiLeaks.

Other spurs to speculation include that Bodo, the town where he was last seen, is home to Norway’s main military air base, that the armed forces’ joint operations center is nearby, and that part of the country’s cyberdefense center is hidden in a mountain outside town. Investigators have ruled out any connection between the military presence and his disappearance.