Margaret Fleming, 36, was reported missing by her two carers in October last year from her home in Inverkip, Inverclyde.
The final independent sighting of her was actually at a family event on 17 December 17 1999.
Police forensic teams spent months sifting through the house Ms Fleming lived in for any documentation of her life and a large garden at the property was excavated as part of the search.
The carers moved out of the property and no clues as to Ms Fleming’s whereabouts were found by police as the search was extended to nearby water and woodland.
Police Scotland confirmed two people had been detained in connection with the investigation.
Ms Fleming is thought to have lived with her father in Port Glasgow before he died in October 1995.
She then lived with her grandparents and her mother, but later moved in with carers in 1997 and has been estranged from her mother since.