There aren't any "clandestine gigs". In my experience, once you have passed through a scanner, the screen the operator views is there for you to view also. You can see exactly what they see - a very generic outline of a human figure, with any area the scanner picked up generically highlighted for them to further investigate. They are not secret "undressing" machines. While not a fan of the whole security rigmarole, they are an effective tool for non-invasively checking that someone is not concealing something in or beneath their clothing that would not have been picked up by a traditional walkthrough metal detector.