They were only alerted to the situation when a parent, who had arrived to collect their child, spotted the pupil alone on a busy road nearby.
A source told the Daily Record the rest of children and staff were all inside the nursery building at the time child was 'wandering' outside alone.
They said: "The staff inside the nursery were not aware that he was lost or left behind.
"A parent arrived to collect their own child and found this other child wandering in the road.
"They returned the lost child to the care of the nursery and reported the incident to the care inspectorate.
"Under duty of candour rules the nursery should have reported the incident to both the care inspectorate and the lost child’s parents immediately.
"I am unsure if staff confessed what had happened or tried to hide it, but I know the person who found the child reported it to the Care Inspectorate."
Care Inspectorate Scotland confirmed they were aware of the incident and said they were working with the nursery to insure children are cared for safely.
A spokeswoman for the Care Inspectorate said: “We are aware of the incident, and we continue to liaise with the service provider to ensure that the safety and wellbeing of children is paramount.
"Every child in Scotland has the right to good quality, safe care which meets their needs and respects their rights.
"Anyone with a concern can contact us on 0345 600 9527."
Angela Elizabeth Nursery declined to comment.
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