Sometimes the airline security contractors — as part of an order to meet a search quota or if suspicious about a passenger for some other reason arising during an initial interaction between the contractor and passenger — will manually select a person to be subject to SSSS type screening despite no SSSS flag appearing on the boarding pass.

Having an SSSS boarding pass on a US-bound flight is not a requirement for being subject to the same kind of screening (or even more) as those who may have an SSSS boarding pass for the very same US-bound flight from abroad. While having an SSSS boarding pass for a US-bound flight is the most probable way to experience the kind of screening you noted , it’s not the only way to be subject to the same type of screening experience.