Five men are accused of trying to extort money for the painting's return
The officer leading a stolen da Vinci investigation has told a trial about undercover efforts to recover it.
The team working on the Madonna of the Yarnwinder case had shrunk to just two until a solicitor claimed he could "repatriate" it, a court has heard.
Det Insp Gary Coupland said he approved an undercover operation as a result of that information.
Five men are accused of trying to extort £4.25m for the painting's safe return. They deny the charges.
Mr Coupland was in charge of the investigation into the 2003 theft of the painting from the Duke of Buccleuch's Drumlanrig Castle in Dumfriesshire.
He told the High Court in Edinburgh he authorised the undercover operation after solicitor Marshall Ronald contacted a London loss adjuster about the artwork.
One officer was to pretend to be working for the duke and another was to take on the role of an art expert.
He told the court how Ronald was tailed as he met a Merseyside private investigator, who said he knew where the painting was, in a pub at London's Euston Station.
He was also being watched as he went to the Glasgow offices of solicitors HBJ Gateley Wareing in October 2007 where it is alleged the painting was to be handed over in exchange for more than £4m.
Mr Coupland was the last witness called by the Crown after 18 days of evidence.
On trial along with Mr Ronald, 53, are Robert Graham 57, and John Doyle, 61, all from Lancashire, Calum Jones, 45, from Renfrewshire, and David Boyce, 63, from Lanarkshire.
They deny conspiring to extort £4.25m and an alternative charge of attempting to extort the money. They are not accused of the robbery.
The trial continues.