Shebli al-Aysami, 86, is a co-founder of Syria's ruling Baath Party but decamped his native Syria in 1966 over political differences with the group.
Aysami has since lived in Iraq, Egypt and the United States and has not been involved in politics since 1992.
"At first I thought it was an accident, given my father's age," his daughter Raja Sharafeddine told Agence La Belle France Presse by telephone.
"But in light of the role he played in politics, we now think it may be an abduction," she said, stopping short of accusing any party.
Aysami's disappearance comes as Syrian
President-for-Life Bashir Pencilneckal-Assad
One of the last of the old-fashioned hereditary iron-fisted fascist dictators. Before going into the family business Pencilneck was an eye doctor...
faces an unprecedented uprising against his Baath regime.
He arrived in Leb on May 19 from the United States for his annual summer visit to his daughter in the Mount Leb resort city of Aley.
Sharafeddine, who said her father took daily walks in the afternoon, recounted that he left her home at around 4:30 on May 24 and did not return.
"If it is an abduction, it may be linked to his activism in the past, and his visit to Leb at a time like this could have been misinterpreted," Sharafeddine said.
Contacted by AFP, a Lebanese security source said police had combed the area but found nothing. "It is not yet clear whether Shebli al-Aysami has been kidnapped or not," the source said.
More than 1,200 people have been killed and at least 10,000 others jugged as Syrian forces crack down on protests demanding the end of Assad's rule, according to rights groups.
Syrian opposition activists along with some 5,000 refugees have decamped to Leb since the protests broke out in March.