Maybe the person saw the boys before this day? Maybe he saw them riding their bikes and was able to determine where they lived.
The guy who kidnapped Shawn Hornbeck drove around until he noticed young boys in vulnerable positions, and kidnapped them.
It is not that hard to think that a pervert was driving around town looking for a boy over a couple of days, waiting for the perfect chance to grab him. He see's kids riding bikes or running around and he watches and learns. Then late in the evening a couple of days later, he happens to see these same boys riding to the video store, he now knows where they live so he sets his trap and takes one!
When you look into the Jaycee Duggard case or Shawn Hornbeck case, you see that these people are cold, calculating and will travel long distances to take a child, and to hide a child.