In the early days of scanners, a colleage and I passed through one at Tokyo Airport. We hadn't seen one before but we passed through without incident. It was only afterward that I realised he had carried an aluminium case containing his electronic engineer's tools through the scanner without setting off any alarm. We discussed this, and concluded that the system wasn't working. Instead, we had noticed that a beady-eyed security official was intensely watching everyone passing through the scanner. Presumably, if he noted anyone appearing to be the slightest bit nervous, then they would have been stopped and searched.
Recently, I've had the eyelets on my shoes set off the beeps. The security guy agreed that the sensitivity of the scanner had been set a bit high, and actually apologised.