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, Inverclyde, on October 28.
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 enquiry 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.
“However, we cannot rule anything out at this point and officers will continue to follow up enquiries about where Margaret has been. I must stress that this is still a missing person investigation.
“Our search will continue from the house into the grounds around it. This is a natural progression in the inquiry to assist our investigation to find Margaret.”
Ms Fleming, who is believed to have learning difficulties, is thought to have attended James Watt College in Greenock between 1996 and 1998, and officers are keen to hear from anyone who remembers her from that time.
Mr Livingstone added: “Margaret’s carers have said that they believe she has a private side to her life and that she may have friends that they are not aware of.
“We are currently working with very limited information which makes this a challenging investigation.
“If you know or knew Margaret, please get in touch with the investigation team based at Greenock on 101. Any small piece of information may be able to assist us.”
Ms Fleming is described as 5ft 5ins with a heavy build, collar-length black hair and brown eyes.
When reported missing on October 28 she was said to be wearing a green tartan fleece or jumper, dark-coloured jeans or trousers, dark Karrimor boots and was carrying a satchel-type handbag.