There was some speculation that Helen might have run away due to her home life:
Anne Martin, a teacher who formed a “close relationship” with Helen’s older sister Maria after the disappearance, told the inquest she heard disturbing allegations of abuse.
“(Maria) told me their father was fiercely violent at times towards them … they were both very afraid of (him),” she said.
Maria, who was 12 at the time her sister vanished, and who committed suicide when aged 18, claimed Helen was “preparing to run away and had wrapped her Christmas presents early”, Ms Martin said.
Not only did Helen have problems at home, she reportedly also had to deal with predators in the local neighborhood, according to this article:
Maria had also told [Ms. Martin] that two boys in the complex, one named Johnny, had raped Helen.
But Maria had also said Helen was preparing to run away and had disappeared at the same time Johnny left the housing estate.
Ms Martin said she had failed to convince Maria to make a formal statement about her allegations.
But if Helen did run away, I have a hard time believing she could have remained out of sight for very long, never mind 24 years. She was only ten after all.