Many pet owners believe that keeping pets indoors means that they are safe from getting lost, or that a microchip alone eliminates the need for a pet ID tag. This is a common assumption, but it does not always match the reality that veterinarians see every day.
All it takes is one open door, a startled pet, or a well-meaning guest who does not realize there are pets in the home who cannot go outside. This is a common occurrence for indoor pets; they slip out unnoticed, and the next thing you know, they are gone.
This is why veterinarians continue to recommend visible pet ID tags, even for indoor pets. An ID tag is often the first form of identification that a good Samaritan will look for when a pet is found.

