That is a GREAT idea like an ICLOUD chip that you can "find my phone".......FOR ANYONE that has a cat and uses the "magnetic" pet doors reconsider. If you do the research, cats have an exceptional "homing instinct" it is thought via magnetic frequencies but magnets CAN interfere with that instinct....HERE'S A DEVICE THAT INTERESTING:
Loc8tor Pet Cat Tracker - Cat Homing Device | Loc8tor for Pets | World's Best Pet Trackers