The 42-year-old, who is the best-known of Fidel Castro's reputed eight sons - full name Fidel Castro Diaz-Balart - was last heard of on 17 June. That was when the island's official newspaper, Granma, reported that he had been dismissed from the once-key job of head of Cuba's Soviet-designed nuclear energy programme. End of official story.
There have been no sightings since, neither on the Caribbean island nor in Spain, land of his forefathers, where Little Fidel, a Soviet-trained nuclear physicist, is long reputed to have maintained an intimate friendship with the daughter of an industrialist.
The only further clue came last month when Fidel Castro senior, after visiting his father's birthplace in Galicia, north-western Spain, was asked about young Fidel by Spanish journalists. He had indeed been dismissed, 'El Comandante' confirmed, adding cryptically that the reason had been 'his inefficiency'. He would not be drawn further.
On Friday, however, an independent and usually authoritative Russian newspaper, Nezavisimaya Gazeta, said it could reveal all. Little Fidel, it said, had in fact been placed under what Cubans call 'pyjama detention' - house arrest - for embezzling dollars 5m ( pounds 2.6m) of state funds destined for the formerly Soviet-backed nuclear energy programme. He was likely, according to the Russian paper, to face a public trial and possibly the death penalty.
The report was gleefully picked up by Miami-based news media catering to anti-Communist Cuban exiles, but officials in Havana declined all comment. 'There has been no public pronouncement on this case since that of June, when the dismissal of Fidel Castro Diaz- Balart was announced,' a Castro spokesperson said. 'We have no further news.'
Nezavisimaya Gazeta did not stop there. There could be only two possible reasons for the detention, it added: the Cuban leader, in Saturnian desperation amid political and economic isolation, felt forced to 'devour' his own son to save himself, or the arrest was made without his knowledge, possibly by the leader's brother and Defence Minister, Raul Castro, as part of a plot to undermine the old revolutionary hero and force him to stand down.
The latter conspiracy theory has been going the rounds in diplomatic circles for many months. There was even speculation that Fidel senior's trip to Galicia last month was part of a move by the Galician Prime Minister, Manuel Fraga, to encourage him to retire gracefully in the land of his father, where he was formally declared 'an adopted son'.
'Fraga wants to show Fidel Castro a possible location for a hypothetical exile,' a Madrid-based Cuban opposition figure, Carlos Alberto Montaner, said before the Cuban leader arrived. Even Mr Fraga himself appeared to hold out the possibility. 'Certainly the Spanish government has said it would welcome him, and Galicia is the home of his parents,' the popular Galician leader was quoted as saying.