According to several sets of statistics, most child "abductions" are variants of familial abduction where a non-custodial parent or other family member takes the kid because they don't like the custody agreement. Sometimes it's mixed nationalities, or mixed religions, other times it's just a really messy and bad divorce. A lot of them are well meaning, in that the person taking the kid has real (at least in their mind) concerns about the kid, and are trying to protect them. Others are spiteful; one side is trying to punish the other by keeping the kid past the time for return on a visitation, or denying visitation. (Yes, I've had someone want to make that report because the other party was a whole 5 minutes late...)
As to prevention... there are limits to what you can do. Schools have, unfortunately, become quite used to specific rules about who can pick up kids but that only helps while the kid is at school. If there's a visitation order, and there are concerns about potential abduction, it's imperative that the family work with the courts to modify the visitation orders such as requiring them be monitored, stipulating where they may go, or eliminating the visitation, depending on the extent of the concern. It's not always easy, though! In Virginia, for example, there still remains a very strong presumption in the courts that "children of tender age" should be in the custody of the mother. There's no definition for "tender age"... I've heard horror stories of mothers selling themselves for drugs, with documentation, the kids wanting to live with dad, and the court still granting the mother custody.
You also have to make sure that the kid understands who they're allowed to be picked up by, and when. Otherwise, they may innocently aid in their abduction by getting in a car with grandma or Uncle Bob... And the kids need to know what to do if a family member like that tries to take them somewhere without permission.