Police are following up a possible sighting of Rory Blackhall walking away from his primary school after being dropped off by his mother.
Police said Rory Blackhall's body was found under a tent
The 11-year-old was found dead three days later in woods about half a mile from his school in Livingston. He had been asphyxiated.
The sighting was made by a cyclist and is one of several being checked by detectives investigating the murder.
Police also want to trace a blonde woman waiting at a bus stop nearby.
Rory's mother Michelle dropped him off near Meldrum Primary School last Thursday morning. He walked through an underpass and into Westwood Park but he did not attend school and was reported missing that afternoon.
His body was discovered in woods on Sunday afternoon. It had been covered by an old tent and his school rucksack was missing.
At a news conference on Tuesday, police appealed for information about the tent and the rucksack.
They say they have had a "tremendous" response from the public to their requests for information and are urging anyone with information to be patient if they have to wait to get through.
A dedicated email address has also been set up. It is email@example.com.
Bus stop appeal
The cyclist contacted police after hearing the appeals for information.
He had been cycling along the south footpath of Deans Road East in Livingston opposite Meldrum Primary at 0840 BST last Thursday when he saw a boy fitting Rory's description and carrying a rucksack. He thought it was odd that the boy was walking away from the school.
The cyclist then spotted a woman, who is only described as having long blonde hair and wearing denim, waiting at a bus stop.
A double-decker pulled into the stop but the woman did not get on, which the cyclist also thought was strange, police said.
The underpass which Rory walked through on Thursday morning
They want to trace her as she may also have seen the schoolboy walking along Deans Road East.
Detective Inspector Tom Martin said: "Whilst the informant is not 100% certain that the boy he saw was Rory, this is the first possible sighting we have of him.
"What is vitally important is that we trace the blonde woman waiting at the bus stop as she should have noticed the boy walking past her as well if he had carried on his route."
More tests are being carried out to determine whether Rory was smothered or strangled.
Rory was not reported missing until about seven hours after he was dropped off to go to school, when his grandfather came to pick him up.
Irene Matier, a member of the national executive of the Association of Head Teachers in Scotland, said: "Communication is all-important here.
"We need our parents to let us know if their child is not coming to school, but we also need to be vigilant in the schools."
She told BBC Radio's Good Morning Scotland programme: "We don't want to over-react about all of this, but I think it is probably time for schools to revisit their procedures.
"Best practice would be the school that does contact parents if a child does not turn up at 9am."
Officers are continuing door-to-door inquiries
In reality, schools often did not have enough staff to contact all the parents of children who may be missing on a Monday morning, she said.
Some pilot schemes were in operation with a view to detecting truancy. "It may now be the time to look at that from a security point of view as well," Ms Matier said.
But she said problems could be compounded on the first day of term when some children might still be on holiday and new teachers were dealing with unfamiliar children.
Meanwhile, Livingston MSP Bristow Muldoon welcomed an increased police presence at a time when parents were frightened.
Mr Muldoon said: "Certainly I've seen myself many police officers in and around to provide reassurance to parents and to children."
Two special assemblies are to be held at Meldrum Primary School on Wednesday to remember Rory, one for younger pupils, the other for older ones.
A further event will be arranged at the school in consultation with is parents.
Anyone with information should ring police in Livingston on 01506 445624