HONG KONG, April 29 (Reuters) - Alarmed by the spread of a new swine flu virus, airports around the world have rushed to install temperature scanners to pick out the sick, but the microbe is proving too clever for modern technology. Experts say an infected person can easily pass through these heat sensors without detection as the incubation period for influenza ranges anywhere between one and three days. "The scanners won't pick up everyone (with flu), especially if they are too early in the infection ... People who have been infected very, very recently wouldn't show up on the scanner,"...