A garden is now being excavated in the search for Margaret Fleming, 36, who was reported missing by her two carers from her home in Main Road, Inverkip, Renfrewshire on 28 October.
Police say they are keeping an open mind but believe she “may have come to some harm”.
Detectives from Police Scotland’s major investigations team have established that, apart from her carers, the last independent sighting of Ms Fleming was at a family gathering on December 17, 1999.
Officers say she has few contactable friends or family and checks on education, employment, health and welfare records have shown no trace of her.
It is believed Ms Fleming lived with her father in Port Glasgow before he died in October 1995.
She then lived with her grandparents and mother but moved in with carers in 1997, and has been estranged from her mother since.
Specialist search teams have been “carefully sifting through the house” where Ms Fleming is believed to live with her two carers to find any documentation, clothing and personal items that could help in the search.
Specialist search teams including the police helicopter, dog unit and dive teams have also been searching across Inverclyde.
Police said the search of the house in Main Road will now extend into the garden and will “involve some excavation work”.
Detective Chief Inspector Paul Livingstone said: “Apart from her carers, the last independent sighting of Margaret was in 1999.
“This, along with facts such as having no trace of a job, few friends that are contactable and no evidence of her contact with partner agencies and local services, has led me to become very concerned for her whereabouts and wellbeing.
“One line of inquiry that I now must consider is the possibility that Margaret may have come to some harm.
“It’s important that we keep an open mind to all lines of enquiry but the distinct lack of information about Margaret is concerning.”