He pointed out that in 1984, 34 Old Masters prints disappeared from the BMA, though all were located within a month.
And as recently as 1988, a security guard stole 145 pieces of Asian art from the Walters Art Museum. All were recovered within a week of the theft's discovery — but not before the thief had melted down several ancient gold objects.
Now, there are art-related research tools and databases that would have been inconceivable in 1951.
"Given the thoroughness of our current record-keeping," Mannix said, "it's highly unlikely that anything like this could happen today."
Anne Craner Norton of the Potomack Company, who authenticated the Renoir landscape, said she did "everything we could possibly do to make sure this painting wasn't stolen before we put it up for auction."
She checked the London-based Art Loss Register, which maintains the world's largest database of stolen and lost artworks, along with the FBI's art theft website.
After she found that the BMA had sold a more significant work by Renoir — a nude — at Christie's auction house on May 16, 1990, for $444,691, she scoured auction catalogs to determine that "Paysage Bords de Seine" had never been put up for sale.
"I don't think that very many people even knew that the painting was missing," Norton said. "That's what's so puzzling to us all."
Wittman confirmed that Norton and the Potomack Company appear to have done their due diligence. And if the criminal investigation from 1951 sank without a trace, it's because at the time, the theft of even a painting by Renoir was considered a relatively minor crime.
That's especially true, he said, because the tiny Renoir appears to have been the only artwork taken in the theft.
Had there been other statues or canvases that disappeared, "they would have been listed on the same police report," Wittman said.
"Probably the report was taken by uniformed police officers. Chances are that it wouldn't have been reported to the Baltimore Police Department's detective bureau because there was no aggravated assault or forced entry. And there were also no leads."
Nor would the theft have been recorded on any of the three major art databases in the world because they weren't created for nearly three decades after the theft. Wittman said that in 1951, art theft wasn't even a federal crime unless it involved the transportation across state lines of goods valued at a minimum of $5,000 — or more than twice the price of the missing Renoir at the time.
"The police could have looked at this crime like it was light shoplifting or something," Wittman said. "It's only been in the past 30 years that the value of art escalated and some pieces began to sell for prices in the hundreds of millions of dollars."
Indeed, the Baltimore museum used the $2,500 collected from the loss of the painting — roughly eight months' wages for the average U.S. family in 1951 — to purchase a modest little bauble: a self-portrait by Edgar Degas.
Baltimore Sun reporter Tim Wheeler contributed to this article.