I have a MedicAlert necklace; you'd never know unless you saw me naked, because I tuck it under my clothing. Paramedics are trained to look for MedicAlert tags in the form of either bracelets or necklaces.

As for subdermal chips, bollocks to that idea. You may be quite happy to ave something jabbed under your skin - I'm not. As any vet could tell you (or for that matter, anyone who's ever had an piercing), subdermal implants don't necessarily stay put - they can migrate under the skin or even "grow out". My cat's had to be done twice when the first one popped out of a cyst on his back (it was originally implanted into his neck). And that's without going into issues over allergic reactions or the whole privacy issue.