microchips in horses are the same as microchips in dogs. Only good if the person/rescue/auction/seller/humane society/slaughterhouse, etc or whom ever has the horse/dog has a scanner to read the chip.
I don't imagine many places will have a scanner to read the chip or take the time to check a horse. Many many rescues, humane societies, etc don't check dogs for chips, I don't believe the majority will even think to check horses. Plus, if the chip is not registered by the owner originally who puts the chip in, there will be no paper trail to contact anyway.