The tragedy is thought to have happened before Christmas as neighbours who forced their way into the flat in Rhyl, Clwyd, on Wednesday found the bodies in the lounge, where presents were unopened and decorations in place.
Tests on the body of the boy, Michael Walker, who appeared to have been playing with toys, showed that he had died from dehydration through lack of care and neglect. But the post- mortem examinations on Tony Walker, 28, and his wife, Elizabeth, 23, were inconclusive.
The Home Office pathologist, Dr Donald Waite, has asked for further forensic tests to help him determine the cause of the deaths of Mr Walker, a postman, and his wife.
However, Det Sgt Philip Welsh told the inquest that there was evidence of drug abuse at the first-floor flat above a hairdresser's salon.
"The bodies were in some state of putrefaction and decay, and there was evidence at the scene of probable drug abuse," he told the inquest at Prestatyn. "Syringes were scattered about the floor and there were powders and items we would usually associate with the drugs scene." He also said that the flat, which was visited last Friday and Sunday by police who found nothing untoward, was locked from the inside, lending weight to the theory that no one else was involved in the deaths.
"There was no sign of violence to any of the three bodies involved and we are not looking for any other person in connection with this," he said. "There were a few small items out of place. I can only assume that the child had been playing with some items. There were a few toys on the floor."
David Jones, the coroner, adjourned the inquest to await a further police report on the outcome of forensic tests.
The townspeople of Rhyl, a fading seaside resort of grand Victorian houses, were shocked that the deaths could have gone undetected for so long.
Joan Butterfield, a councillor, said the local authority had long recognised the problem caused by the division of large properties into flats and bedsits, which gave rise to a highly transient population.
"Where there is a predominance of flats we have a moving populace very similar to the kind you find in a university town. In this situation people are less likely to know their neighbours. Perhaps this is reflection of the way we live. People are less neighbourly," she said.