I got SSSS'd recently on HKG-SFO. Regular security didn't care, but they did at secondary security on the jetway. For US-bound flights from HKG, they have secondary security just prior to boarding where they go through everyone's carry-ons to check for liquids (even airport-purchased liquids are not allowed, above 100ml). Because of the SSSS, they also swabbed my carry-ons and my person and ran the sample through a portable detector machine. They had the selectees printed out on a clipboard and were checking them off as they were screened. No special procedure upon arrival at SFO, although I do have Global Entry. Even so, I tend to think that oldmoster's experience above with CBP was unrelated to the SSSS.