they are metal detectors - they can be adjusted to detect how dense the metal is. that is why if you leave the stuff in your pockets where its more spread out, it doesnt go off, but if you hold it altogether the detector sees the metal as more dense. basically your cell phone being more dense than say a penny, the penny wont go off, but the phone will due to density alone.
the problem with BGW is they have theirs set so high up that they aren't really detecting much.
IF programmed properly, the detectors should go off for things such as cell phones, keys, even the foil on cigarette boxes can set it off or the chip in your credit card.
BGW is terrified of the the crowds these things were going to cause, so they try to set them so they dont hardly ever go off and just have people walk through with everything; in theory the lines should move quicker, but what BGW failed to realize is guests arent stupid. they know the procudre. empty the pockets into a bowl, and the whole process wouldnt take very long if they ever properly trained or bothered to implement the system proerply.