I've been gifted with SSSS 3 times in the past 3 years out of HKG, Although it was really only a minor hassle. Last time the secondary screening happened after scanning BP in the same area they used to do the bottle check on the US bound flights. They had me remove shoes and swabbed my hands, shoes and bag. Took less than 5 minutes. Other 2 times was similar experience. Each time I got SSSS, check-in agent apologized and told me it was random.