Mystery surrounds the whereabouts of a Chinese billionaire reportedly taken away from his Hong Kong hotel by mainland police, in a case that has parallels with last year's disappearances of five booksellers.

Chinese police officers escorted Xiao Jianhua from his suite at the luxury Four Seasons hotel last Friday, according to overseas Chinese news sites that carry reports of political gossip and unverified corruption scandals.

Xiao is the founder of Tomorrow Group, a well-connected financial services company.

Hong Kong police say their initial investigations showed the "subject" crossed into the mainland at a border checkpoint on Friday. They received a request for assistance from a family member on Saturday but a day later the relative asked the request be dropped after receiving a report that he was safe.