My brother found an emaciated dog a few years ago with a scab all the way around his neck from an embedded collar being ripped off, and massive burns/scarring all over his body. When we took him to the vet, he was found to be chipped, but we never called the number on the chip. We posted fliers around (Found small male dog, call to identify), and posted the same on Craigslist, and never heard anything. AFAIK, there isn't a way to re-register the chip without contacting the old owners, so we simply left it and we make sure he always has an ID tag on. I know that's not 100% effective, but the microchip company said that there was nothing we could do without proving the transfer of ownership.

I couldn't imagine soemone mistreating this little guy!