Even indoor only cat can get out and subsequently lost. This can occur when someone leaves the door open or during disasters such as house fires, tornados, hurricanes, floods and robberies. The changes of indoor only cats that get lost of being reunited with their owners is estimated as being as low as 7%.
Although ideal, most indoor only cats don't have collars with ID tags. This makes it especially important that indoor only cats have microchips. That may be your only chance of being reunited with your cat.
For more information on microchipping your indoor cat, please read Microchipping Your Cat.