There are conflicting reports about Xiao, 46, with the South China Morning Post saying he had been led away by unidentified people and other sources suggesting he had been abducted by local Triad gangsters, who had been hired by the Chinese police.
The paper said police examined CCTV footage and did not find any sign of a scuffle involving Xiao or his female bodyguards.
Hong Kong's Chinese-language Apple Daily reported that his own spokesman was denying he had been abducted and maintained he was in mainland China "on holiday".
It said Xiao had been taken over the border between Hong Kong and Shenzhen in mainland China with his wife on Friday, who then returned and reported the case to police and media.
She then said Xiao had contacted her and did not want to exaggerate the incident.
The Hong Kong Free Press said the Tomorrow Group posted a message on its WeChat account yesterday, saying Xiao was taking a break because he was ill.
The message said: "Regarding the reports on me in recent days, I have to say that I, Xiao Jianhua, have been recovering from a sickness outside the country.
"Currently everything is fine," it said, adding the company was running "normally".
It was later removed.
It is unclear why Xiao may have been targeted, but it will add to fears that Chinese authorities are increasingly interfering in semi-autonomous Hong Kong.
Xiao, who reportedly had links to the family of China's President Xi Jinping, was one of several mainland businessmen who lived at the Four Seasons Hotel.
Hong Kong police confirmed they had received a "request for assistance" concerning Xiao on Saturday.
Xiao is reported to have come to Hong Kong in 2014, two years after Chinese President Xi started a much-publicised anti-corruption drive.
He has denied he was ever the target of an investigation.
The Chinese-language news site Boxun said Xiao had been tipped off that a special investigative unit was about to abduct him.
Xiao reportedly came from a poor village in Shandong but went to university in the late 1980s, where he was a student leader at the time of the Tiananmen Square uprising. Later he used his intelligence and knowledge of capital markets and leverage to build a financial empire in banking and insurance.
The Ministry of Public Security in Beijing has not commented on Xiao's whereabouts but China is in the middle of the week-long Lunar New Year holiday and most government offices are closed.