I'm surprised not more people know about this, so I'm hoping I reach a lot of people with this info. It's about having a secret family password (article in French).
I had one growing up. It's a special way to help reduce the chances of child abduction. The way it works is you and your child come up with a special secret word. You teach your child that if someone other than the regular person who's supposed to take them home ever shows up at school or at an activity and tries to take them home, they must say the password.
This means that even if Uncle Bob, who the child knows well, tries to pick them up, the child must not go with them until the password is said. Since abductions are often committed by family members, this is one way to minimize the risk.
It also means that if you are sick, injured or stuck at work and need to send a family friend to pick up your child, you have a backup plan in place - you can give them the password. I then recommend changing it to something else for the next time.
Don't think your family or friends are so perfect they would never abduct your child. This is everyone's worst nightmare, but it's also somewhat preventable.