The Argentine Navy informed the US Naval attache, early on April 2nd, 1982, "one or more" Soviet submarine were near the islands. The US attache briefed BRITNAVAT Buenos Aires about the subs, and he sent the message to MODUK Navy via Flash Message.
So, from the very beginning, the Soviet subs were part of the equation.
In fact, the war was triggered by a Soviet boat. Mr Victor was detected and HMS Superb was deployed, from Gibraltar to chase the contact (I´ve the pic of the same Mr Victor, taken from a Nimrod). The press reported Superb was heading south, to the crisis in South Georgia and the Argentine government said now or never (a nuclear sub was a no go for the assault).
From there, many suspicious submarine contacts were reported. The book cover many of the mistery contacts and, specially, the May 5th, 1982 attack by a Tracker and Sea King of a submerged contact, far away from HMS Splendid, the closest UK boat. They dropped two Mk.44 SW torpedoes against a solid contact, detected by sonobuoys, MAD and Sea King passive sonar. Almost, almost the same story told by Adm Parry, which happened a couple of days later.
However, the mistery remains. I could say (quoting myself from Ewen Southby Tailyour´s "Exocet Falklands") a friend of mine was on ISA meeting Odessa 2004 and some Russians told him they were there (below water) waiting for "the order" from Moscow. And a Russian amateur historian informed me K-525 (an Oscar class boat) was down south in 1982. True or false, who knows. Some info I have support that claim, but the Soviet files of the time are still closed (and yes, I´ve asked via official channels).
Regards!