SSSS = Security Search for that Special Someone. It's part of the "random" profiling that will grant you additional screening at the security checkpoint. Certain behaviors, such as one-way tickets, last minute purchases, etc., will get you there.

I had to deal with flying with no ID when one of my colleagues lost his wallet when we were traveling for work. Same procedures - you go through the "puffer" machine and/or the wand/pat down, and your entire content of the carry on luggage is searched through.