My understanding is that the US requires random screening at the boarding gate, on departures from overseas, which is paid for by the carriers. They normally use 2 or 3 screeners, and I'm told their instructions are to "keep busy". The norm is to grab the first few people who board, meaning they ensnare a lot of Elites and Premium cabin types, which is pretty silly, since they are more likely to be known to the airline (and more likely to get grumpy, as they watch casual customers pass by, unmolested). I just wait until all of them are busy, and then board. One would assume a reasonably intelligent "bad guy" could come to the same conclusion.
