Ms Lane, 33, was devoted to her son Steven, according to her family who said mother and son were "inseparable". But as she battled with psychological problems, and the strain of losing custody of her older son, Ms Lane apparently broke under the stress.
Steven - now being cared for by foster parents in Dorset - was found under bushes in his blue track suit on Monday. He was asleep and shaded from the bright sunshine at the spot where his mother left him three hours earlier, clutching a hold-all. She has not been seen since despite a nationwide appeal.
Dorset police thought she had made contact when they received a telephone call from a distressed-sounding woman on Wednesday, but the call turned out to be a hoax. The woman who made the call has been traced and reported for obstructing the inquiry, police said last night.
Despite her dedication to her son, Ms Lane's plight was apparently that of a depressive whose desperation was not heeded. She had shown signs of growing strain over the last 18 months, after she moved to Hemel Hempstead in Hertfordshire from Milton Keynes when she split up with her boyfriend, the father of her older son.
The police repeated their appeal for Ms Lane to come forward last night. Inspector Glen Chalk, leading the investigation, said: "We ask Julie to come forward so she can be reunited with her son. She can be assured she will be treated with the upmost sympathy."
When her father, George Lane, 62, who lives in Northamptonshire, saw the pictures of Steven on the television news he was shocked to recognise his own grandson. He received a call from his daughter at 4am on Monday, who told him she had spent a lovely weekend at the coast. He speculated she had "taken a bad turn", and that the depression had returned.
Neighbours at Plantation Way in Hemel Hempstead say Ms Lane's odd behaviour had intensified since she moved there. She dyed her hair orange, was allegedly aggressive on one occasion, and they would hear Steven screaming for hours on end without being tended to, prompting neighbours to call social services and the police.
Lesley Clark, 40, contacted social services at Easter when she heard him crying for five hours from behind the net curtains of the terraced council house. She was convinced on one occasion that Ms Lane had barricaded herself into the bedroom when she heard furniture being moved around, and she would worry when she did not Steven for days at a time.
Ms Lane also allegedly told neighbours that her older son, Tony, eight, had died. He had been very ill four years ago, and needed open heart surgery, she said, which led her to have a nervous breakdown. But Tony is alive and well in Portsmouth where he moved with his father after she lost the custody battle for him 18 months ago.
An inquiry will be held by Hertfordshire social services into Ms Lane's case, but officials denied they had failed to see the danger signs. They also insist she was offered support including help to find Steven a nursery school, which he last attended on Friday, where his progress was carefully monitored.
Liz Railton, Hertfordshire's assistant director of social services, said: "We did take action. We visited on every occasion when neighbours reported their concern. While we would routinely look at any case like this to make sure we did respond, support was clearly given. From our perception, it happened out of the blue."