The us government requires a certain amount of secondary screening for flights into the US. In the us they do this more via the beeps when you go thru screening where they then say you have been picked for random additional screening or something like that, outside the us it is more the ssss but it has nothing to do with boarder control and zero impact on GE. Based on my own experience I would say it is over 20 percent of the time I get secondary screening when coming from LHR, never had a GE issue.