The way things are TODAY, if a boy leaves CYF without being properly released, the County Police would be notified immediately.
It is hard to say what the situation was back in 1973 regarding kids running away from Boys Village. AND it would be hard to say what police would do even if they were notified right away. They might not have looked very hard and even when a dead boy is found, might not link him to the Boys Village Cheltenham (now called Cheltenham Youth Facility or CYF).
It is possible that IF the boy was at Boys Village, he might have been placed there afer being picked up for some infraction or crime and might not even have his real name. If he was picked up with no Identification, no known relatives in the area, etc,- and he could have given a false name. He would be kept there until an adult parent or guardian could be located.
IF he were to have run away, Boys Village might not have had much information to report. And if police were to have gone to Boys Village with photos of the dead boy, it is possible that the people they showed the photos to had not actually seen him.