Prime Minister Prayuth Chan-ocha, the military chief who engineered the 2014 overthrow of Yingluck's government, also said he did not know where she was, and the government was "looking for her".
"I want those who wish to support me to listen to the news from home, to avoid risking any unexpected problems that could arise from those who have ill-intentions toward the country and all of us", she wrote, without naming anyone.
Yingluck left Thailand on Wednesday - two days before a verdict was due to be delivered in a trial over rice subsidies - and is now "safe and sound" in Dubai, the source said. "She asked for sick leave not to show up today".
Norawit Lalaeng, her lawyer, could not confirm on Friday that Ms Yingluck was still in the country, and attributed her failure to appear in court to an "ear fluid imbalance", according to Reuters. "(The verdict delay) provides a window for potential flight.if she has not fled already", he said.
Yingluck has said she was only in charge of coming up with the policy but not the day-to-day management of it.
Her brother, Thaksin Shinawatra also a former premier, fled Thailand in 2008 before he was convicted of graft and handed a two-year prison term.
Throughout her trial Yingluck defended the scheme, saying she acted in good faith to raise the incomes of the poorest and was the victim of "a subtle political game".
"What?" said a 75-year-0ld woman when told the court considered Yingluck to be a fugitive. However, the court was not provided with medical documentation and believed she intends to flee.
Last month, Yingluck herself openly acknowledged her need for public support, writing on her Facebook page, "I would like to transform your moral support into a power that would make me strong and tolerant".
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Both siblings remain popular among the rural poor, but are hated by an urban and middle-class elite.
Under the rice subsidy program, Yingluck's administration paid rice farmers up to 50% more than market prices.
Shortly before the military coup in 2014, a Thai court ordered Yingluck's removal from office, finding her guilty of violating the country's constitution in her reassignment of a senior security official in 2011.
The youngest sister of Mr Thaksin, she was seen by her opponents as a proxy for her brother, who was controversially ousted by the military in 2006. Yingluck's supporters believe the case is politically motivated.
But 3 years of repressive junta rule has successfully quashed any widespread opposition to the military for now.
Mr Shinawatra founded and bankrolled the Pheu Thai party, which has won every Thai election since 2001.
Farmers did make more money, but critics say the subsidies led to billions of dollars in losses for Thailand, along with enormous stockpiles of unsold rice.
Yingluck has pleaded not guilty while prosecutors allege staggering losses of $17 billion to the state resulting from the scheme.