Metal detectors at the airport only go skin deep. Same with the full body scanners.
When going through the big scanner thingies I generally look back at the screen when exiting them and my hands are noticeably red-er than the rest but then the security persons seems to completely ignore it. I guess they just see normal hands and that’s enough to know there’s not a gun hiding there.
Actually they can detect ferrous metals anywhere in the body. The ability to detect small objects depends on the sensitivity settings on the scanner… which is usually not that high because it’s looking for big things like guns.
Yep this is because millimeter wave radar used in these machines does actually penetrate deep enough to bounce off the implants just under the dermis, but it’s right at the edge of the wavelength and the signal deteriorates considerably going through the skin twice… so the machine can tell something is there but the indicator on screen is on your hands, and the people running the machine clearly can see your hands when you put them up over your head and you’re not holding a weapon, so meh… they assume medical implant and let you pass.
I’ve had the scanner actually put red boxes on my hands before and they did stop me and ask me if I had any medical implants… I just said yes and they let me walk.
I spent most of the winter travelling across Colombia, Ecuador and Peru via Miami and Vegas, originating on the West coast U.S. I flew about every other week for six months and I had zero problems with the implants anywhere, no one even asked about them or ever seemed to notice. I have a blue xsiid in L2, an xdf2 in R0, and a NeXT in R2.