ON THIS day 25 years ago: a 35-year-old woman, Ranko Nakano, was detained and was subjected to an investigation by the Japanese Government about her background. She was caught with 11 other people trying to land at Castle Peak from Guangdong. Mr Yoshinobu Nagashima, a Japanese vice-consul, said he had sent a report to his government seeking to confirm her claim of Japanese links after a brief interview with her. ''She could hardly speak Japanese,'' Mr Nagashima said, adding she conversed in Cantonese throughout the interview. The woman claimed she was born in Guangdong and had never set foot in Japan, but she said her father was Japanese. She also said she was married, but ''was alone'' in the escape bid. The woman did not give any reason why she made the attempt, and she was ''excited during the interview, presumably due to a change of circumstances'', Mr Nagashima said. THE ''Great Revolutionary Alliance'' in Guangdong province was officially proclaimed in Guangdong, said several arrivals who took late slow trains from that city to Hongkong at night. The alliance, if officially confirmed and consolidated, was a ''great success'' for Mao Zedong in south China, said a local political observer. But there was no mention of the establishment of a revolutionary council in the regular broadcast by Radio Guangdong. The arrivals said they heard broadcasts over street loudspeakers that half a million people would take part in celebrations throughout the day, and saw groups of people marching towards the Yuet Say Shan Stadium in the city before their departure. They said General Huang Wing-sing, the military commander of Guangdong province, appeared to be the leader of the alliance as his name was repeatedly broadcast in the streets.