The inquest also heard that Noah could have been “confused or disorientated” inside the drain which was described as “pitch black”.
Turner also recorded a negative result for the presence of drugs in his body.
The pathologist also said the boy’s death could have taken place within hours of his disappearance.
She said she believed the schoolboy “was dead for quite a period to time before he was found” and it was likely to have been “closer to the time he was last seen”.
Turner said she did not believe Noah would have floated down the tunnel and she thought he was likely to have made his own way down.
Two other expert witnesses, the former State Pathologist for Northern Ireland Professor Jack Crane and forensic pathologist Nathanial Cary, also gave evidence on Tuesday.
Professor Crane said Noah could have become disorientated in the drain “not knowing how to get in or out” while Professor Cary said a “cold confused space” would have been a very difficult environment for Noah.
All three witnesses agreed there was no evidence of any third party involvement in Noah’s death.
