SSSS happens once in maybe 7 flights for me from YYZ. It's annoying, but the Canadians take it fairly seriously. No Nexus security, full body scan or pat down plus thorough going through of security. At YYZ, you get a special CATSA (their version of TSA) stamp if you have SSSS. If you do not get this, you will be very much screwed at the gate and sent back to security. I learned this the hard way once.