TWO senior Hong Kong officials visiting Beijing to boost cross-border co-operation on aviation matters have apparently gone into hiding after their unpublicised trip was discovered. The Secretary for Economic Services, Gordon Siu Kwing-chue, changed his hotel as soon as he finished a phone call from an unwelcome Hong Kong journalist yesterday morning. He is in Beijing to attend the signing ceremony of an agreement between Hong Kong and mainland aviation authorities on the training of air traffic controllers. Director of Civil Aviation Peter Lok Kung-nam, who will sign the agreement on behalf of the Government, checked out of his hotel late last night. The trip, planned weeks ago, has been soured by the Sino-British row over the airport project. Mr Siu dropped earlier plans for the visit after receiving an ''invitation'' from members of the Preliminary Working Committee to brief them on the Airport Corporation Bill. He is believed to have arrived in Beijing on Sunday night, while sessions of various sub-groups of the PWC were still under way. He declined to give details on his trip. The Government later confirmed Mr Siu's visit, but asserted it ''has nothing to do with an earlier invitation'' from the PWC. The timing of the return of the two officials has not yet been fixed, but is expected to be either tomorrow or Thursday. When confronted at his hotel in the afternoon, Mr Lok said he was in Beijing to sign the accord but declined to say when and where it would take place. Mr Lok refused to receive any phone calls and when found at the Yansha Hotel before dinner, said he knew nothing about Mr Siu's trip. ''We have different missions in Beijing. I don't know his mission here. I don't know where he is staying. I don't know whether he will attend tomorrow's ceremony. I don't know when he will to return Hong Kong. ''But I'll try to reach him tonight,'' he said. Mr Lok said he would only sign the accord for the training programme, which related strictly to aviation technology. He said he would not meet the PWC to discuss the new airport. Shortly after 9 pm, the hotel said Mr Lok had checked out. A spokesman for the British Embassy, Janet Rogan, said she did not know the whereabouts of Mr Siu. An official from the Hong Kong and Macau Affairs Office said it was not involved in the aviation co-operation scheme and had no plans to meet Mr Siu. Civil Aviation Administration of China officials declined to give details on the signing ceremony. Under the training agreement to be signed by Mr Lok, three Chinese staff will make up an initial batch who will learn about Hong Kong's air traffic control system. Hong Kong officers will later travel to Beijing to get first-hand experience of the mainland system. Discussions have started for the airports in Hong Kong, Macau and Shenzhen to work more closely in co-ordinating flight information. A regional co-ordination centre may eventually be set up. Hong Kong and mainland aviation officials have stressed that the training programme is merely a technical exchange programme without political overtones. ''As we have said before, the Hong Kong Government values co-operation on economic matters with China, particularly in technical fields such as civil aviation,'' a government spokesman said. ''The agreement to be signed [today] is particularly important since there will be at least four airports operating in the Pearl River Delta within a radius of [160 kilometres] when the new airport at Chek Lap Kok becomes operational.'' The ceremony would have been held last week, but the controversy surrounding whether Hong Kong officials should appear before Preliminary Working Committee (PWC) members forced the two sides to reschedule the event.