If they aren’t recovered how do you know it was a non-custodial parent?
It's reasonable to assume that the mix of situations doesn't change too much. It would be unreasonable to assume that every run-away always returns / is returned, every non-custodial parent is always forced to return the child to the custodial parent. So, there will be run-aways and non-custodial parents in the group that are abducted.
Where are you getting the idea that 200-400 are stranger-abducted?