A 50-YEAR-OLD mental patient who twice evaded security at Kai Tak airport last month has apparently committed suicide near his home in Kwai Shing, Tsuen Wan. Police denied they had been in the process of prosecuting the man after reports that two summonses relating to the airport incidents were found in his flat. ''We said all along we were not going to prosecute him,'' a police spokesman said. ''I do not think any summonses have been issued.'' Chan Sing-foo was found dead on Saturday night after apparently falling from his eighth-floor flat in a temporary housing estate in Hoi Shing Road, Kwai Shing. Chan prompted a top-level Security Branch investigation after he twice breached security procedures. He was initially found by engineers aboard a Cathay Pacific 747, after slipping past unmanned immigration desks and boarding an apron bus on March 17. Police drove him home. Three hours after his release, he was found wandering on the apron after bypassing checkpoints. He was later sent to Kwai Chung hospital for treatment.