Disturbing for sure. Inquest may give some clarification on the time of death but Donal doesn't appear to know yet. Thats kind of unusual isn't it, they must have some idea. Decomp, core temp would give a rough estimate. Not disclosing that, witnesses hearing voices, police searching the area points to him being alive in there possibly on the sun night anyway.
This might boil down to not getting police on the ground, overtime budgets, this attitude thats its another teen gone AWOL who will turn up eventually. There are similarities here with the teen Matthew, again there was no urgency there, he possibly could have been saved.
The greenvale tragedy, police on scene who left.
I wouldn't focus so much on how or why Noah entered the drain, things happen, sometimes they're difficult to explain. The disturbance in the area on the night he disappeared, the witnesses and when the police had a hunch is probably important.
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At a guess I'd say that's what the PII is covering. Intelligence that he may have entered the drain on sun evening and not enough was done, they've possibly dismissed it.