I think those machines can be very picky sometimes. Today my body set off the full body scanner at YVR; I went through a second time, set it off in even more places. Not sure if 'set off' is the correct term - an image of my body appears on their screens, together with yellow rectangles that I guess they have to investigate. They asked if I was wearing a necklace, anything, and I said no, I was literally just wearing one plain cotton t-shirt. No undergarments, no cards, no phone, no implants, no prosthetics, no nothing. And I said that, and they said we know (I think they could see), but we still have to take you in a back room and do a body check. In the back room, I said, look I don't mind I can take my t-shirt off and you can see it's just my rolls of fat that's too dense for the machine to scan, and they (I think the body patdown requires two people present by regulations) said, no we have a procedure, don't worry. So they patted me down and took residue swabs from my hands and jeans waistband.

I have also had my hair tied in a ponytail set it off, and they tell me it's not uncommon, and ask me to untie my hair and rescan. I think any unusually dense bits can be flagged. In the scanner the second time today, they told me I needed to straighten out my body more, to stand up tall and look upwards more, but I guess I was slouching too much to please the machine.