SAN FRANCISCO, Jan 30 (Reuters) - Thousands of people were evacuated from a San Francisco International Airport terminal on Wednesday after security screeners detected possible explosive residue on the shoes of one passenger, who vanished before he could be questioned. Airport spokesman Mike McCarron said the possible security breach occurred shortly before 7 a.m. (10 a.m. EST), when a man going through a random security check tested positive for some kind of explosive on his shoe. "They screened his shoes by doing a trace wipe on them. The wipe came back as a possible positive hit of residue of some type. We don't know what yet," McCarron said. "When they went to ask the gentleman some more questions he was not there anymore. He was lost in the crowd." Officials immediately ordered the entire boarding area cleared, shutting some 30 gates in the airport's Terminal 3, the main domestic gateway used by both United Airlines and American Airlines. In addition, passengers who had already boarded a number of jets were told to get off and go through security screening procedures again. The Federal Aviation Administration ordered airports to perform random shoe checks in late December after passenger Richard Reid, 28, allegedly attempted to set fire to a pair of explosive sneakers on an American Airlines flight traveling from Paris to Miami. As thousands of people milled in the chilly morning air outside the San Francisco terminal, police and airport security officers swept the building for any sign of the man, described only as a white male in his 40's. McCarron said it was unclear if the man who set off the security scramble was aware that his shoes had tested positive for possible explosive -- noting that exposure to nitrates in anything from heart medication to firecrackers could raise red flags in the screening process. "He may not have even known they wanted to talk to him ... but he was lost in the crowd rather quickly. It's a very busy time here at the airport," McCarron said. Airport officials estimated that it could take several hours for traffic to return to normal, noting that the entire building would be checked before the thousands of waiting passengers could be sent back through security.