Short or a locable bracklet that can be taken off or necklace only other on i could think of would be a actual implanted microchip that has a GPS tracker in it.

We use a dog collar tag that has a GPS got from petsmart. Works in general but inly because the tag looks like one of his necklace tags so he doesnt notice anything different.

There are ones that are built into insoles. Quite expensive at £249. Good peace of mind though.