GUWonder

Regarding: "Also some countries can require that X Percentage/number of passengers be randomly selected. This accounts for a large number of SSSS on the flight from that country, but PRECHECK on the connecting flights. TSA decides if you get Pre. However the foreign government can still require you be a selectee. I get this flying through LHR with VS/DL periodically. Even now that both are PreCheck airlines."

There are PreCheck outcomes for flights connecting outside of the US on non-US carriers? That would be news to my eyes if so; but I've seen no evidence of that. And so I read the above words to be about travel to the US. The only "foreign" government requiring "this" (i.e. "that X Percentage/number of passengers be randomly selected" on travel to the US is the US. That's the party to which a complaint about this should be directed if it keeps bothering someone on their trips.

I am pretty sure that South Africa still doesn't tell DL or VS "that X Percentage/number of passengers be randomly selected" for extra screening on flights to JNB. But you're welcome to hit up the South African government to see if "this" is still the case. Otherwise, there's no point in complaining to the South African government about a gate search at LHR; instead concerns about such flagging issues should be addressed to the government that drove the flagging issue and the airline which checked you in and/or is operating the flight with such search; those are the parties that are most capable of reducing the chances of this hitting again.

For flights to the US from LHR, it's the US and operating carrier that drive the flagging. The UK (as in the government) doesn't have a rule "that X Percentage/number of passengers be randomly selected" for flights from the UK.