Just read this thread. We adopted two kittens and had them chipped, etc. Fitted SureFlap microchip flaps and they worked perfectly. At 11 months old Jenny went missing so we notified Petlog, local rescues, papers, radio, put up posters, etc. all to no avail.
A little later Jake started having problems getting through either flap so they were left unlocked while we were still searching for Jenny, then a letter came from the vet asking us to bring the cats back for re-chipping as they had been using a faulty batch...
So Jake now has 2 chips (One fails to read intermittently) and Jenny?....Doubt we'll find out, her code is still on the flaps just in case.