That's pretty much the only consensus about the theft of Picasso's 1937 painting Weeping Woman from the National Gallery of Victoria in August 1986.

Rumours have swirled around Melbourne in the decades since about who was behind the theft. While the painting was returned 16 days after its theft was discovered, the culprits have never been caught.

Victoria Police still considers it a cold case. But could a piece of missing evidence be the key to solving one of Australia's biggest art heists?

Weeping Woman was purchased with great fanfare in 1986 for $1.6 million. Due to currency fluctuations, that price increased to $2 million by the time the final instalment was paid. (Jackson Pollock's Blue Poles, in comparison, was bought for $1.3 million in 1974).

Some estimates put the painting's current worth at more than $100 million. (The gallery does not comment on the value of works in its collection, an NGV spokeswoman said.)

The theft was discovered on a Monday morning, August 4, when an NGV employee noticed the painting was missing from the gallery wall, with a registrar's card in its place stating the artwork had been moved "to the A.C.T.".

It soon emerged that "the A.C.T." referred not to the Australian Capital Territory: rather, it was revealed as an acronym for Australian Cultural Terrorists after the group sent letters to the gallery and to The Age.

"We have stolen the Picasso from the National Gallery as a protest against the niggardly funding of the fine arts in this hick State and against the clumsy, unimaginative stupidity and administration of that funding," the first letter reads. It was addressed to "Rank Mathews", ridiculing the then Arts and Police Minister, Race Mathews.

The group demanded a 10 per cent increase in arts funding and the creation of annual prizes for young painters – and threatened to destroy the painting if their request was not met within seven days. A later letter ominously contained a burnt match.

Police searched the gallery repeatedly (including draining the moat outside and searching the art school, which was then part of the same building) as well as a number of artists' studios during the investigation.

The painting was returned 16 days later, left in a locker at the former Spencer Street station.

Yet items that were stolen with the canvas remain missing. Labels detailing the painting's exhibition history and provenance were not with the painting when it was left in locker 227 on the night of August 20, according to former NGV chief conservator Thomas Dixon.

The labels were encapsulated in clear polyester plastic and attached to the painting's masonite backing board prior to the theft. Neither the labels nor the backing board have been recovered.

In his notes from August 20, 1986, detailing his examination of the painting after its return, Mr Dixon writes: "We have a photocopy of these labels should they be recovered to prove they came from this picture ... presumably, these are still with the thieves."

Speaking exclusively to Fairfax Media ahead of the 30th anniversary of the theft, Mr Dixon, who worked at the gallery for 22 years until 2006, said he wished someone would return the labels and solve the mystery.

"I'm holding out hope that someone come forward and say 'It was me'." He said the loss of the labels wasn't a "game changer" in terms of the artwork's provenance, but they could be crucial in proving who stole it.

Patrick McCaughey, who was director of the gallery in 1986, says he has no new theories about the theft. Yet his comments imply that he knows more than he's letting on.

"One or two people who might have known something about it are now dead ... who may have been able to cast some light on it," Mr McCaughey said this week by phone from the US, where he is now based. When quizzed about who they may be, he dismisses the names I proffer.

"I'll give you my last reflections on this, OK," he says, "and that is, I've always thought there was some likelihood that there was some internal assistance in the theft ...

"I think they knew how to use a registrar's card so it wasn't missed, it wasn't immediately discovered and that A.C.T. could have been seen as the [Australian Capital Territory], not Australian Cultural Terrorists."

Mr Dixon says police investigated a theory involving staff embroiled in a love triangle. "I was so shocked when police showed me a letter written by one of my staffers to a love rival. This person's former partner sent them a ... pretty out-there letter, a lot of sexual innuendo and personal accusations, [saying] this person could have stolen the Picasso."

Neil Holland, a former Victoria Police forensic scientist who now works as a consultant specialising in document analysis (including cultural artefacts such as Ned Kelly's Jerilderie letter and Christina Macpherson's Waltzing Matilda manuscripts as well as suspected forged paintings) examined the letters and the painting after it was returned.

Mr Holland's analysis of the letters – of the typewriter script, the ribbon and paper used – found that they were from the same source. While he documented the painting and the packaging it was found in, and knows DNA was recovered, he's not sure whether police would utilise advances in forensic technology to solve the case.

"I'm certain you would have DNA ... but with the use of that technology, you don't use it on every case," he said. "Probably these days, someone would have to make a decision that this case is important enough to warrant that step, of looking at the DNA."

When an Age photographer went to the gallery on Thursday to shoot the painting, as arranged with the NGV, the Weeping Woman was missing from the wall when he arrived – as part of routine works, we're told, to be returned to her regular spot on Monday morning.

Yet another twist in the endlessly intriguing history of Picasso's painting.

Additional research by Michelle Stillman

Do you know more? Email d.cuthbertson@fairfaxmedia.com.au