I've never been SSSS'ed when I wasn't expecting it. I get hit with it about a third of the time I fly WN, and nearly all of my WN segments are last-minute one-way purchases.

Have you ever been at an airport where the regular security line was a mile long, but the SSSS line was very short? I've been tempted to volunteer myself to be SSSS'ed in that case, but I don't think I'd ever do it: that would probably arouse some suspicion.