This is a true story of a family, a husband and wife with two kids 5 and 3 years old were planning on a trip to Disneyland. Since these kids are energetic and prone to get lost, the family bought an USB drive for each of the kid and one for them. And installed this program "I'm Lost" with a secret phrase on the USB Drive and also added Cellphone numbers on the USB Drives.
These USB drives were left inside the boy's shirts. And the family instructed the boys to cry for help when they are lost and if someone finds them they can give the USB Drive for information to contact mom and dad with the information provided in the USB Drive. And about some minutes after they went to Disneyland, the three-year-old boy went missing. And the parents were worried whether the boy would have followed their instructions and the kid did exactly what he was told.
The boy screamed for help and one person found him and got the USB drive. That person handed the kid and the USB drive to the security, who then accessed the details and called the parents. The boy was saved thus with a simple USB Drive.
The program "I'm Lost" initially was meant to retrieve missing USB Drives only but they also found another use to locate the missing kids also. The USB drives are so cheap, I think any parent can do this for their kids.