Concern for Mikaeel Kular's well-being has grown because of nighttime temperatures near freezing, the Guardian reported Friday.
Officials were uncertain how long the child has been missing or how he was able to leave his home. He was put to bed about 9 p.m. Wednesday and his mother, Rosdeep Kular, discovered early Thursday he was missing.
The apartment building in which the family lives has a locked main entrance.
Police have questioned family members, including Mikaeel's father, who is estranged from the boy's mother.
"No one is detained, no one is a suspect, we have no evidence of any criminality," said Superintendent Liz McAinsh of Police Scotland.
The area in which the child's neighborhood is located is near the shoreline of the Firth of Forth. The search has been conducted by land, sea and air.
Police initiated a United Kingdom-wide alert Thursday, an action usually reserved for kidnappings.