Just a note, I have GE and Nexus, and I have gotten SSSS at YYZ. It's not so bad there, but it is quite important you get a special stamp by the CATSA (Canadian TSA) to indicate that you've had the extra search. Otherwise you won't be boarding as planned. It also triggered a PPBM and multiple extra searchers of my checked bag. When checking in for this last trip, the United agent himself told me that he got SSSS recently...this is despite even having a badge for the airport.

Seems like it just happens randomly...even to trusted travellers

On the return for the same trip, no extra screening in NRT or DEN. My boarding pass didn't have PreCheck printed on it, but I still got the 3 beeps anyway.