Officers carrying out an initial search of a house and its ground, in Motherwell where Ellen Ruffle lived, said it has now led to exploratory excavation work.
As the second day of the search for Mrs Ruffle got under way, police said they were concentrating on one specific area of the property.
A police spokesman said: "There has been an initial search using ground penetrating equipment. That has revealed an area of interest.
"The excavation is under way to establish what may or may not have been picked up."
The exploratory dig is taking place in part of the garden of the house in Annan Glade.
Police said it was too early to say if human remains had been found.
The primary teacher and mother-of-two was 34 when she vanished on November 14, 1969.
As reported in yesterday's Evening Times, police started their search after a tip-off. It is believed officers have been told Mrs Ruffle was killed and her body dumped in the garden.
Police are using X-ray ground scanners and radar machines and a forensic tent has been erected. Specialist teams could be seen searching the garden.
Mrs Ruffle lived at the address from 1967 to 1969 with husband Norman and their two children, Graham, then aged 18 months, and Rosemary, then 4.
Superintendent Andy McKay, of North Lanarkshire police office, is leading the investigation.
He said: "It was completely out of character for her to leave her two small children.
"It may be decades since Ellen was last seen, but my officers are no less determined to find out what happened to her."
Mrs Ruffle, known to friends as Evelyn, was working at Cathedral Primary, Motherwell, when she went missing.
Shortly after her disappearance, her husband took their children to South Africa.
He is understood now to have returned to Scotland and is believed to be helping inquiries.