Only as I read this do I realise that this may be why I set off the detectors occasionally. Some of my office shoes I have rebuilt by the shoe mender (they're nice comfortable Church's; does anyone still use a proper shoemender in the US?) and he puts on stainless steel heel caps because that's where I wear them down, while driving.

In Britain, unlike the US, if you set off the detector you get taken aside and frisked (patted down) straight away rather than having to keep trying the detector, taking more and more things out of your pockets, so it's never apparent what set it off.

A couple of years ago I broke my shoulder (in a skydiving acident) and had a steel pin inserted (now removed). That always set the detector off. I never went to the US while that was in. How do US procedures cope with that?