20 Mar SSSS
I tried to check in for my flight to Boston in advance and, at the very end (after entering all of my info), it indicated that there was an issue and that I would have to check in at the airport.
I arrived at the airport just over an hour before the flight, as with Global Entry it is possible to clear screening and customs pretty quickly. Unless, as I learned, you get the letters “SSSS” written under the Remarks/Observations segment of your boarding pass. It stands for (I learned) Secondary Security Screening Selection. It didn’t turn out to be a huge deal (no pat down), but definitely took a lot longer, as I had to wait for someone to look at all of my bags.
After doing some reading on the policy it seems that the trigger was probably the fact that I had purchased a one-way ticket.