The SSSS flagging -- which I consider to be haraSSSSment flagging and resulting screening -- comes about by two primary means:

1. DHS/TSA kicking back the indicator when check-in occurs -- the indicator kickback is a result of automated hits;
2. Airline/airline security contractor-based selection -- the indicator kickback may involve automated selection but sometimes involves manual selection.

At LHR, there is DHS/CBP's IAP in play also. CBP employees working IAP can lead to passengers being flagged down for gate-area secondary screening at LHR.

Generally the haraSSSSment flagging and screening -- a secondary, gate-area screening of flag-hit passengers at non-US airports -- is independent of CBP secondary searches at US ports of entry.

CBP gets access to airline PNRs, and so anything they get that way can be used in some way.