A man was found dead in his own car that was covered near his house in Chiang Mai's Muang district on Thursday morning.
He was pronounced dead at Chiang Mai hospital. Somphet Thaso, 48, died about two days earlier.
His adopted son, Sukit Somboonpattanakoon, said Somphet could not be accounted for two days and he could not find him until he walked past the car and smelled a strong stench.
He lifted the car cover canvas and saw the body so he called the police.
The car was not locked but its four windows were closed.
Police said there were no traces of injuries on the man and suspected carbon monoxide poisoning. The fact the car was covered might suggest suicide, they added.
But his relatives said his bag with a mobile, an ID card and some cash was missing.