Two decomposing scents were found by a police dog in the garden of the house Margaret Fleming was living in, a court has heard.
Edward Cairney, 77, and Avril Jones, 59, deny murdering her at Seacroft, Main Road, Inverkip, between December 18, 1999 and January 5, 2000.
Ms Fleming, who would now be 38, has allegedly not been seen for more than 19 years.
A huge police investigation to find Ms Fleming was launched in November 2016 which involved a poster campaign and searches with police dogs and divers.
Constable Ryan Galloway, 42, was part of the dog unit which searched inside and outside of the Seacroft house.
He said his black labrador Ollie found two possible decomposing scents in the garden close to the River Clyde.
This was in two cup sized holes dug 18 inches apart and 24 inches deep.
Mr Galloway said: “The dog’s reaction was quite clear as his behaviour changed and he became more focused.
“He tilted his head back and started to bark.
“He is only trained to identify pig and human flesh.”