What mr flappy said.
After being 'randomly selected' again, whilst in uniform, I asked the security person "why do you pick out the crew members all the time". His reply was, "because we have to do a certain number of searches per hour and you guys aren't allowed to complain. It's just easier this way".
Walking through BNE security recently with my wife, she asked me what the security person with the wand does. I told her to smile at the lady and she would find out. She did, and she did.
So if you want to get bomb sniffed at security, just smile at the operator and they'll pick you because you look easier. Frown and scowl and they'll likely ignore you as you pass by. Of course they will deny this, but after the umpteen dozenth time it's happened to me, I'm convinced.