Thaksin could be seen walking out of the private jet terminal of the Don Mueang International Airport from a livestream video on Monday afternoon.
Last week, Thaksin said he intended to travel to Singapore for a medical checkup, but had to change the destination to Dubai instead because he was delayed by Thai immigration.
He said his plane would not be able to land at Singapore's Seletar Airport, which serves small aircraft, before its 10pm closing time.
Thaksin lived in Dubai during his self-imposed exile starting in 2008.
He said he had regular physicians there.
The Supreme Court on Tuesday is set to rule whether officials mishandled his return to Thailand in 2023, to begin an eight-year sentence for three cases involving graft and abuse of power.
At the time, Thaksin was sent to a suite at Bangkok's Police General Hospital, reportedly for medical reasons, after spending less than a day in prison.
His sentence was commuted to one year by King Maha Vajiralongkorn, and he was released on parole after six months.
The circumstances raised questions about whether he received special treatment.
Tuesday's Supreme Court ruling will determine if the Corrections Department acted legally in its handling of his case.
The mandate for the court's investigation is broad and it is unclear if Thaksin could be liable for the Corrections Department's actions, though if he was found to be complicit in their wrongdoing, he might face charges that could result in a new prison sentence.
Thailand's parliament voted on Friday to confirm Anutin Charnvirakul as new prime minister to replace Thaksin's daughter, Paetongtarn Shinawatra, who was unseated after a court found her guilty of an ethics violation for a politically compromising phone call with Cambodian Senate President Hun Sen.
Many see this as a major blow to the political influence that billionaire Thaksin has wielded for more than two decades.
Thaksin was prime minister from 2001 until a military coup ousted him in 2006 while he was abroad.
He briefly returned to Thailand in 2008 but left again, skipping bail ahead of a court verdict on a corruption case.
After leaving office, he faced a barrage of lawsuits and criminal charges he claimed were politically motivated.
In August, a criminal court acquitted him of royal defamation, an offence also known as lese-majeste, which could have resulted in a 15-year prison sentence.
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