I think it is probable that the CCTV time was erratic.
The detective said it was 9 mins slow. But when was that CCTV time checked, against real time?
It was sometime after Richard Donoghue made his first statement where he said he went home, had a cuppa, and then saw FD at about 10:45am searching and calling for William.
What made him waver about his original (and likely most accurate) statement is:
"If what Detective Roberts has told me about the time difference is correct ..... "
Mr Donoghue later saw a man in his yard on Benaroon Drive - who was William's foster father - and that had influenced his belief that he had left the tennis club later than the time recorded on the CCTV cameras.
Did William Tyrrell disappear earlier than reported? Why CCTV could prove crucial | Daily Mail Online
It seems highly improbable to me that all of the people who saw and heard FD calling for William that morning got their times wrong.
Was CCTV time adjustment (or non-adjustment) for daylight savings starting and stopping taken into account?
Did the CCTV auto adjust its time, or did it have to be manually done?
Power outages ... do they affect the CCTV time?
Erratic connectivity in that area ... did that affect the CCTV time?
Just how well did that CCTV clock keep time?
Richard Donoghue was never called to the inquest, so perhaps the police got that little riddle sorted out and it satisfied the Coroner in the police brief. imo