My big horses are both chipped in their necks. I can't remember which chip, I think they are Home Again chips. Last time I had them checked, they hadn't migrated and were still doing their thing.
I don't know if it's the same now, but years ago when I had these horses chipped, the vet told me that each chip manufacturer made it's own reader. If a different brand of reader was used, the info would not come up. Although maybe it could detect that there was a chip in there? I don't know. I do wish there was a universal reader for them though. That's the only thing that's kept me from getting all of my animals chipped. Maybe there is now.
When I was breeding dogs, all my dogs were chipped and their chip # and registration # was tatooed on their inner thighs, just in case. Wouldn't work for horses though I suppose.
Didn't I hear that a law was passed (maybe it was a local/state law?) that all slaughter houses that handled horses had to scan them for chips before slaughter? This also would've been a while back.