Immigration officials are investigating another reported border blunder in which a traveller in transit is said to have bypassed security checkpoints at the airport three times. The incident reported yesterday is the third in the past six months. Autistic boy Yu Man-hon, 16, and Tsoi Wa-kau, 22, who is mildly mentally handicapped, managed to cross the border to Shenzhen without travel documents. The latest incident involved a man who was suspected to be mentally ill and was in transit about six months ago. He allegedly passed immigration and Customs control points unchecked for a tour of Tung Chung new town, where he was intercepted by police. After he was sent back to the waiting lounge in the airport restricted area, the man managed to slip through control points and took the Airport Express to Central, where he was found by MTR staff without a ticket or valid passport. He was then said to have slipped through immigration checks a third time and was caught by Customs officers at the baggage-claim area. The Immigration Department then sent an officer to watch him until he left Hong Kong. A spokesman for the Immigration Department said it was still investigating the incident. The senior manager of Aviation Security Company, Gavin Ho Chi-hang, said it had only heard about the incident yesterday, adding that passengers leaving the airport would not be subjected to their security checks. He said the company's responsibility covered only passengers leaving Hong Kong - making sure they had boarding passes and passports and had undergone security checks. Yu Man-hon dashed through Lowu's immigration control point after running away from his mother on August 24 last year. Shenzhen security officers who intercepted him asked SAR officers to check his identity, but Man-hon was sent back to the mainland side after they failed to confirm if he was from Hong Kong. He is still missing. Mr Tsoi was found at the Huanggang border checkpoint by mainland officers on February 15, a day after his family reported him missing. Immigration officials did not find any record of Mr Tsoi leaving Hong Kong and the case is still under investigation.