There has been a lot of controversy lately about the microchip industry for pets in the US. There is no single registry for all microchip numbers and not all microchips can be read by a single microchip reader. There is also the issue of owners not properly updating their information with the microchip company. Sometimes that is simply an issue of not knowing how a microchip actually works. There is a misconception that microchips have GPS capabilities and will help the owner locate the dog or cat when it is lost. Unfortunately, we are at the mercy of the good samaritan who finds the animal to have it checked for a microchip. Even then, if the owner's information is not up to date, many shelters are stuck having to rehome the animal instead of returning it to the original family.
Here is a heartwarming story about a lost dog, finally reunited with his family after being away for 9 years.
http://time.com/3991072/missing-dog-for-9-years-reunited/