This happens to me from time to time with dogs in the rescue with old microchips. We register chips for adopters, so we try to handle all this and clear the chip so no one is burdened by worry that someone's going to want to take their dog back.
Most of the time, there's bad contact info on file (i.e., the chip company is unable to reach them: letters are returned, email bounces, and phone number is disconnected), or the chip was never registered. If they're reachable, they simply ignore the request because they don't want to be bothered about a dog they got rid of. Every once in a rare while someone expresses relief that the dog they once loved and couldn't care for made it out of the shelter alive and into rescue to have a chance at a happy life again -- those are nice calls because then we can get the previous vet records and find out some backstory.
The typical experience for me is that a week later, the chip company transfers the record over because the prior owner ignored their messages.
Last edited by Magwart; 03-14-2018 at 11:59 PM.