SHIPPING officials fear a sudden switch in ''anti-smuggling'' activity by China after the seizure of a Taiwan-bound container ship belonging to a prominent Hongkong line. Representatives of Excelsior Shipping are trying to secure the release of the 4,943-tonne Thuban, believed to be under armed detention in Zhuhai. The Taiwanese-registered ship's last radio broadcast on Saturday said it had been boarded by armed Public Security Bureau officials just outside Hongkong waters off Waglan Island. Sources said it was unusual for such action to involve such a prominentfirm as Excelsior. Woman robbed A WOMAN working for a Quarry Bay finance company was robbed of $110,000 while on her way to deposit the cash at a bank yesterday. She was attacked by three men while waiting for a lift on the seventh floor of a King's Road building at about 2.45 pm. They snatched an envelope containing the money and fled.