The painting of an unidentified bearded man was found in a bin liner dumped in a loft in Cooks Mead.
Surprised police stumbled across it while on a drugs raid.
Two other paintings found in the property were instantly recognised by the National Art Collection as those stolen from Somerset House in London, a theft which sparked an international high-profile search.
Despite almost unanimous suggestions that the historic painting must be worth a lot of money, art experts including Scotland Yard's Antiques Unit have failed to trace the owner or identify the artwork.
Detective Sergeant Duncan Woodhams, leading the investigation, said all nationally-reported cases of art thefts had been looked at and there were now indications that it could have been taken from another country.
Despite the way it was stored, he said there was no evident damage to the painting and that research had revealed it was not found in its original frame.
Police are currently in talks to feature the artwork on the Antiques Roadshow and Crimewatch to help with the identification.
On Tuesday, 58-year-old Leslie Churchill from Cooks Mead, was sentenced to six and a half years in prison for handling the two stolen paintings that were taken from Somerset House, possession of Class A drugs with intent to supply and money laundering.
He had pleaded guilty to the charges in October.
Detective Sergeant Woodhams said the paintings were found with around 1kg of cocaine, having a street value of around £90,000, and £100,000 in cash.
He said: “We couldn't believe our eyes when we saw them. We thought the quantity of drugs was something special.”
After the sentencing, he said: “Leslie Churchill was a man who made money and gained from other people’s misery. This sentence sends out a strong message to would be criminals in Hertfordshire – we will find you and you will be brought to justice.”
The two identified paintings were Shipping by John Thomas Serres from 1821 and Frances Cotes' 1764 portrait of the Scottish architect Sir William Chambers. They have both been returned to Somerset House.
Anyone with any information about the mystery painting is asked to telephone 0845 33 00 222.