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In the US, getting the dreaded SSSS mark of Cain on your boarding pass has well known consequences, with the TSA singling you out for an extra level of attention.

But what happens when you get one on your way to the US, while you're still beyond the reach of the TSA's loving latex gloves?

As a result of seemingly being put on some form of US list, for a period of time I was given SSSS on my boarding pass every single time I flew into the US (until I went through the DHS Traveler Redress program at which point it stopped). Between that and a few random times, I have received SSSS on my boarding pass for flights into the US around 10 times.

In every case except one I was subject to additional security checks, although exactly what those checks were depended on the airport I was flying from. In general it ranged from a minimum of an explosives swap of my hands and my luggage, through being scanned with a personal metal detector wand, up to them opening and looking at my carry-on luggage. In some cases this was done at the gate before boarding, in other cases it was done in a secure area away from the gate, after which I was escorted to the plane (presumably so I couldn't reclaim something I had left in the airport before the checks?)

As a result of going through the process so many times, I also witnessed multiple other people going through the same process - and (again, depending on the airport) it was clear that there was a level of profiling going on as a part of deciding how well to search people. I'm a white male, and I complied with the requests of the security staff, so I was generally given a very cursory check. In some locations other people were clearly given more thorougher checks, including for example a having many of the items in their carry-on luggage removed and inspected.

The one exception to this process was at Ben Gurion Airport in Tel Aviv, which already has sufficiently good security for all passengers that no further checks are carried out if you are SSSS'ed.

At least in Sydney, the answer appears to be very little. Checking in online worked, security/immigration was normal, the only difference was that at the gate I was taken aside and had a quick explosive sniff test done. This is virtually identical to the one regularly done in Australian airports anyway, the only difference being that they "sniff" your hands as well. The procedure took about 30 seconds and then I was on my way to the Land of Security Theater the Free.

I've had SSSS once in Canada (at Regina, YQR). I got a bag search at the gate before boarding, and a quick pat-down. It only took a couple of minutes. I had no extra screening happen when I passed through security.