The comments around Russian subs is very interesting – I have just finished reading blind man’s buff about the US intelligence use of Subs. In one of the later chapters there is considerable discussion of USS Parche a sub fitted out to monitor russian undersea cables.
At the time of the falklands the USS parche was going on a mission and this was went via the cape, aparrently due to a spy in the CIA, the russians were aware and posted hunter killers to try and catch and monitor the Parche, many being located in the south atlantic. So it is a least plausable that the Argentinian Carrer spotted one of these if it was not one of the “two” uk subs in the area