Question about markings by Document Clerk
 

The last time I flew, the document clerk did the usual scribbling on the pass, and also scribbled something that looked like SSSS. I thought, OK, a new adventure. But then nobody asked to see my boarding pass again and I proceeded through the checkpoint with no difficulty.

Was that a real SSSS, or have the document clerks just been told to put some kind of mark on it that can be used to tell they marked it, and not a passenger? Or was it something else?

They check the required items on the ID to the BP. When done they initial it to say, "I checked all of these." Really, it is that simple.