Hahaha. Thanks. Makes sense.
Pacemakers can. The operators know about them.
Actually, the instant I tell the inspectors that I have a pacemaker, they pull me out of the queue and do the hand thing. I think they can detect the pacemaker too, although I have never asked. And as I said earlier, there is the metal plate in my ankle.
My wife had a full knee replacement about a year ago. Oddly, it never sets off the metal detectors.
Mine was recently replaced due to a drained battery. The instructions for metal detectors for the new one say to try walking through metal detectors normally. If it goes off, tell them you have one. An inspector seemed upset at me when I did that though.
Actually, I have had the same experience. Mine was replaced in 2022 and came with the same instructions. The older ones said not to go through airport inspection and the inspectors have been trained to respect that. But the newer pacemakers seem immune to being damaged by the inspections. Still, it seems easier to just tell them I have a pacemaker.
Responding to my own post to say I spoke too soon. Last night we went to a concert and the metal detector at the door was set off by my wife’s knee.
According to my cardiologist, it is possible to do an MRI now. They have to program it, though, so the guy from the ICD company would have to be there. Doctors in other specialties don’t seem to know about it yet.
The bits in my knee are titanium and it’s a coin flip whether they’ll set off the detector.
My anecdotes. Sometimes, my belt buckle will set off an airport metal detector. However, without the belt, everyone will get to see my tighty-wighties.
My wife went through a “Bullwinkle” x-ray and she got held for secondary scanner. I turned around in time to see the silhouette outline on the machine and it highlighted where she has tattoos. Another time, the metal detector alerted on the metal studs in her jeans pockets. Sometimes, I wonder if she just likes getting felt up at the airport. 
An AC electromagnetic field will be affected by any metal so no, it does not need to be ferromagnetic. They probably have a sensitivity setting.
My husband has a screw in one tibia, and it’s never set anything off.
I did get patted down once between the knee and thighs - no metal, but apparently my cellulite looked dangerous. This was back when the full-body scanners (vs metal detectors) looked dangerous.
I went through a scanner the other day at an ER entrance and had my car keys dangling from a carabiner on my belt look - lots of metal. It didn’t blip at all.
I have a hard time imagining a tattoo ink that would show up on a metal detector. Maybe millimeter wave scanning? I don’t know many details about that.
I think that tattoo inks have iron in them. I’ve heard that people with tattoos can’t use MRI machines. I don’t think my wife has had that problem.
Inks might very well have iron compounds in them. But most iron compounds are not ferromagnetic, and almost none are conductive.
But how awesome repairs would if they were - just stick that little screw on you bicep & you’ll know exactly where it is when you go to reassemble!
**WINNING! **
I’ve never seen a metal detector at a hospital. Big city?
Suburb of a big city but well out into the suburbs. The metal detector there was clearly a) new, and b) not working.
We took my son to a visit at a Kaiser Permanente location last year and they had them there too.
If TV medical dramas are to be believed, roughly 20% of ER visitors are packing guns or knives!
Tattoo inks are made from small particles of pigments. The particulate form matters as this is part of what stops the tattoo from being absorbed or broken down. Tattoos don’t dye the skin tissue. They pigment it.
Black and brown pigments may be made from iron oxides. In particulate form, these can be magnetic. Not much different to magnetic tape coatings.
Magnetite is near black in colour.
Maybe a fast changing gradient field in an MRI could drive the particles through their magnetic hysteresis loop enough that energy is dissipated in the particles. So perhaps they could heat up enough to be uncomfortable, but one suspects this would be something of an unusual occurrence.
Abby Zwerner still has a bullet in her from when she was shot in her school by a 6yo student