Police found 596,000 methamphetamine pills hidden in a car left unattended on a soi in Bangkok's Don Muang district.
No arrest was made as no one arrived at the car parked on Soi Chaeng Wattana 14 in Thung Song Hong sub-district.
Deputy national police chief Gen Chalermkiart Srivorakhan said officers spotted the car at 5am on Tuesday following a tip-off that a large drug shipment would be delivery to Bangkok.
He said police staked out the car until 6:30am on Wednesday but no one appeared so officers opened the car to check and found the drugs.
Metropolitan Police Commissioner Lt Gen Sutthipong Wongpin said officers got information about the shipment after an arrest of an alleged drug courier, Akkarapol Channgkorn, 31, on March 22. He was arrested inside a deserted factory on Soi Vibhavadi 80 in Don Muang while allegedly delivering 20,000 meth pills.
Police will check security cameras to find out who parked the car, the chief said.
The licence plate showed that it is owned by a woman in Nakhon Pathom and police would summon her for questioning, a source said.