Mr Sombath disappeared from the streets of the capital of Laos, Vientiane, after he was pulled over at a police checkpoint.
The disappearances continue in the region: from a Cambodian teenager last seen covered in blood during a labour protest, to an ethnic minority activist who vanished in Thailand after confronting national park officials.
The case of Sombath, an award-winning champion of sustainable development but one who avoided direct confrontation, horrified civil society in Laos, a one-party communist state slowing emerging from decades of isolation.
His disappearance on December 15, 2012 stood out partly because the evidence pointing to abduction was so compelling -- and also because a stream of international figures called for his safe return including Ms Hillary Clinton, Mr John Kerry and Archbishop Desmond Tutu.
"The crime of disappearance is particularly cruel," Mr Sombath's wife Ng Shui-Meng, a soft-spoken Singaporean who lived in Laos with him, told AFP in Bangkok.
"It's very difficult to live with the unknown."
There are no hard numbers. But dozens, and likely hundreds, of people have vanished across Southeast Asia in the past two decades, often after coming up against local business, criminal or political interests.
In Thailand alone, there are at least 81 open cases of enforced disappearance dating back as far the mid-1990s, according to Ms Angkhana Neelapaijit from the Asian Federation Against Involuntary Disappearances.
Others have been killed, apparently silenced in retribution for their work to raise community grievances, including two activists shot dead last week in Thailand.
In the Philippines, relatives of 58 people killed five years ago in the country's worst political massacre are still waiting for justice. No one has yet been convicted and witnesses have been murdered.
Ms Angkhana's husband Somchai, a human rights lawyer, was last seen being taken into custody by police in Bangkok in March 2004. His fate remains unknown.
"There are many others in the Asia region who have disappeared but their mishaps were never mentioned or received any attention," she said this week at a news conference in Bangkok.
One case that has largely flown under the international radar is that of Mr Por Cha Lee Rakcharoen, a Karen rights activist also known as Billy, who was apprehended by national park officials in Thailand on April 17, ostensibly for illegal honey gathering.
He was en route to meet fellow ethnic minority Karen villagers to help them file a lawsuit accusing authorities of torching the homes of 20 families in Kaeng Krachan National Park in 2011.
The local park chief says he was the last person to see Billy alive, but a police probe has yet to declare a suspect.
In Cambodia, 16-year-old Mr KhemSophath has been missing since January 2014, when security forces opened fire on striking garment workers near an industrial park in southwest Phnom Penh, killing four.
Rights groups say Mr Khem, a factory worker, was last seen by friends lying on the ground, conscious, but with blood streaming from what they believed to be a gunshot wound to his chest.
Diplomats and rights groups in Laos told AFP the last time the authorities gave an update on their investigation into Sombath's disappearance was June 2013.
CCTV cameras in Vientiane captured the moment his battered jeep stopped at the police checkpoint.
But Mr Sam Zarifi, from the International Commission of Jurists, describes the investigation from the Laotian authorities as "wholly inadequate".
The official apathy has had a chilling effect on activist groups and civil society, rights groups say.
Many now steer clear from controversial subjects such as land seizures or environmental damage caused by companies, local officials or powerful individuals.
Mr Phil Robertson from Human Rights Watch said the "silence" from the Laotian government is a tactic "favoured by an increasing number of governments in the region".
They are using disappearances "to instil fear in communities wanting to raise grievances against the authorities," he said.
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