Three-year-old Noah Woods had left a play area "through a gap in the fence", an inquest in Ipswich has heard.
The toddler was found dead in a lake on the afternoon of Tuesday, September 15, following a huge search in Brantham, Suffolk.
Search efforts drew droves of volunteers to the Village Hall to support the desperate operation to find the youngster, who police said was non-verbal, partially deaf and had been undergoing an assessment for autism.
More than 1,300 people joined the search over the following 24 hours.
He became separated from a relative after visiting the playground in Merriam Close, and his death was not treated as suspicious.
An inquest into Noah’s death was opened and adjourned in Ipswich today.
Detective Chief Inspector Matthew Connick told the hearing that on September 15, Noah, of East Bergholt, Suffolk, was being "looked after by relatives and this was part of a normal routine for Noah".
He said they walked to an under-fives play area and CCTV footage in the park "showed Noah playing with his relative".
The first CCTV footage was timed at 2.13pm and he was last see playing at 2.37pm, said Mr Connick, who added the CCTV "is motion-activated so doesn’t provide continuous footage".
He said the relative, who was not named at the hearing, had provided a statement to police.
The officer said Noah "left through a gap in the fence leading to the memorial garden where he left through an open gate".
He said Noah went in the direction of Decoy Pond and he was "being closely followed by his relative who was trying to keep up with him".

He said the relative "shouted after him", but after 200 metres Noah could not be seen.
Mr Connick said: "Police were called and a large-scale search operation began."
He said Noah was found by a Met Police diver "approximately five metres from the bank and within submerged tree roots".
He said the depth was estimated at 1.8 metres at that point.

Mr Connick said that a post-mortem examination at Addenbrooke’s Hospital mortuary in Cambridge recorded his medical cause of death as pending further investigation.
He said Noah’s death was confirmed at 4.31pm by paramedics.
His identity was formally confirmed by his mother Charlie Peck at Ipswich Hospital that evening.
Assistant coroner for Suffolk Jyoti Gill said at the outset of the hearing, which was not attended by family: "I would like to express my deepest condolences to Noah’s family and friends and all those affected by Noah’s death."

She said at the end of the six-minute hearing: "The inquest is adjourned to a date to be fixed in due course. Once again, my deepest condolences."
At the time, Superintendent Tom Pearse said: "I want to once again extend our thoughts and deepest sympathy to Noah’s family and thank them for their patience and understanding throughout the searches.
"It was clear from the media coverage we saw that they not only had all of Suffolk behind them, but an outpouring of goodwill from across the entire country and beyond."
He added: "I know how grateful Noah’s family are for all the support and kind messages they have received, but would reiterate our plea to now allow them the time, space and privacy they need at this deeply distressing time."

In a post on Facebook following his son’s death, Noah’s father, Rhys Woods, described his son as "the sweetest and most innocent little boy, with nothing but love to give".
He wrote: “We are going to miss him so very much.
"This was the most heartbreaking outcome for us.
"We kept hoping that he would be hiding somewhere obvious and that someone would find him. That’s what we told ourselves throughout the search."

Head of Investigations