It is only because of the FinCEN files, a huge leak of confidential information, that the world knows a sliver more about the predilection Roman Abramovich apparently had for buying economic rights in players at clubs other than his own. This nugget has been sifted from more than 2,000 suspicious activity reports (SARs) filed with the US government’s Financial Crimes Enforcement Network that would have remained unknown but for the reporting on the leak by the BBC and other media organisations.

Yet as these episodes always do, by providing a one-off glimpse into vast secretive dealings, this leak also delivers a jolting reminder of how much remains unknown about the multibillion-pound business of human football talent.

Years pass before tiny pieces of this huge global jigsaw turn up accidentally, and are painstakingly put together. The BBC reported that the Chelsea owner also owned a company, Leiston Holdings, registered in the British Virgin Islands, a tax haven, that was advised by the agent Pini Zahavi and bought rights in players at Sporting. A spokeswoman for Abramovich did not respond to the Guardian’s question about whether this report was accurate, but in a statement concentrated on emphasising that there was no wrongdoing in any of it: “third-party ownership” of footballers was allowed worldwide until Fifa finally banned it in 2015 – the Premier League banned it earlier, in 2008 – so no football rules were breached.

Abramovich and his advisers could not know if a SAR was indeed filed with the FinCEN relating to him, she said, and if it was, “millions of SARs are filed yearly and a filed SAR on its own does not mean that a transaction was unlawful or otherwise in breach of rules or regulations. To further questions about why he invested in third-party ownership, Abramovich has declined to comment, and there is no requirement for him or anybody else to provide further information.

In all these years of enormously billowing transfer fees, eye-watering commissions paid to agents and profits made by investors in tax havens from buying and selling rights in players, solid facts and details have largely remained confidential. The FinCEN material about Abramovich owning Leiston is a revelation but most of the jigsaw puzzle remains in shadow.

One small piece had been lying about in public for years: it was a long-established fact that Leiston had bought rights in Sporting players but it was not known who owned the company, because it was registered offshore.

So the world is now informed that Abramovich, one of the original Russian oligarchs, while owning and vastly funding Chelsea, also bought up rights to Sporting players. They included the Peru winger André Carrillo, who played against Chelsea in the Champions League in 2014. Zahavi, the agent whose long career is a one-man atlas and history of the burgeoning transfer business, has confirmed that he advised Leiston on players whose rights it might buy, although he said he had not known the owner was Abramovich.

It was already known that Abramovich, through Chelsea, was apparently involved in third-party ownership. The Guardian reported six years ago that Chelsea appeared to be in partnership with the Quality Football Ireland group of companies that invested in players’ rights, advised by another of the game’s prodigiously-earning agents, the Porto-based Jorge Mendes. Then, too, it was Sporting players – nine of them, including Ricky van Wolfswinkel, the Dutch striker Norwich signed for £8.5m – who could be identified as having their rights part-owned by these companies.

There is a reason why Sporting came up repeatedly as an apparent base of third-party ownership by Abramovich-owned or -linked investment entities and others: like Benfica at that time, they declared such deals in their annual financial reports. Sporting’s accounts, for example, note that in the year to 30 June 2014, the club owed Leiston €2.6m for “amounts already received for the transfer of part of the economic rights of some players”. Quality Football Ireland was also listed, owed €14m, along with Doyen Sports Investments, another well-known company then involved in third-party ownership, owed €4.5m, and Holdimo SA, owed €20m.

Away from those Portuguese clubs that published their dealings with third-party ownership investment companies, the extent of the trade at other clubs was mostly not declared. This device, in which a club sold a percentage of a player’s value for cash up front from an outside investor, was institutionalised through the 2000s in Portuguese, Spanish and South American football. It was alien to the English game before 2006 when the investor-owned Carlos Tevez and Javier Mascherano signed for West Ham immediately after making their names with Argentina at that summer’s World Cup.

The Premier League’s revulsion at aspects of those deals and at the concept of players’ rights and transfer fees being owned by investors rather than clubs led to the ban in England and pressure on Fifa for the global ban that came seven years later. But the legacy of this practice, who made what money on which players, remains largely unknown outside these occasional leaks. And the global transfer business, the value of which has now multiplied into fees unthinkable even in 2015, is still conducted through dealings and documents that remain confidential, with the public allowed barely a peek.