Autism Lost and Found Stats?

I have read that when those with an autism spectrum disorder who are found after wandering off are often found very close to their last known location. News stories seem to confirm this fact a lot of the time. Just yesterday, Shelby Wengerd was found one block from her home. Does anyone know the source of this flight risk statistic?

There is a 25% chance that they are found within 4/10 of a mile of where they went missing from and a 50% chance that they are in within one mile of disappearance.