Russians steal secrets of submarines, it is still unknown what exactly is

In 1982, after the completed combat patrol over the Falkland Islands, the British submarine committed an almost mythical act of theft - they stole a secret sonar in front of a Soviet warship.
The role of the "HMS Conqueror" in Operation Barmaid points to a larger world of collusive intelligence services of the Cold War far from the eyes of the public. "HMS Conqueror" was a nuclear submarine of the Churchill class and one of the most powerful ships in the British Royal Navy in the 1980s.
During the Falkland war, the Argentine Navy "ARA General Belgrano" sank, when it was found that the ship was a threat to the operational labor of the Royal Navy during the war. It was only the second time since the end of the Second World War that a submersible torpedo was sunk by some ship. The first time this happened in 1972 when the Pakistani submarine "Hangor" sank the Indian frigate "Khukri".
Just two months later, "Conqueror" was nine thousand miles away, in the Barents Sea. It was equipped with an unusual set of tools: a pair of remote controlled scissors for cutting steel cables and television cameras. All in the interest of stealing a top Russian traction sonar.
There are two types of sonars: active and passive. An active sonar emits short sound waves, known as pings, who travel through the ocean before bouncing back to the boat that made them. On the other hand, a passive sonar simply listens to the noise in the ocean, listening to suspicious sounds such as active sonar pings and noise of machines.
Passive sonar can be difficult to use effectively, as the ship often has to fight its own noise, especially the noise that propellers produce through the water. As a result, the passive sonar is often dragged a mile or more behind the ship.
It is, according to The Telegraph, a tactic like the Soviet Union started in the 1980s. And since the device did not broadcast any noise, almost no Americans knew about the device, and they could not learn anything except to put it on the table and split. So the United States and Britain decided to steal a sonar. The plan was to enter the disguised Soviet spy ship under the Polish flag, and simply cut the cable that the sonar was attached to the ship.
The plan was complicated for several reasons.
First, "Conqueror" had to intercept an intelligence journey - which may have happened in international waters. Disclosure would imply immediate and perhaps deadly retaliation.
Secondly, the Royal Navy submarine had to carry out the operation while both ships were moving. Third, the submarine had to avoid recognizing through sonars trying to steal.
According to "The Telegraph", "Conqueror" did this and went on the run with an stolen device. Divers later surfaced and fetched the stolen sonar and transferred it to the home of Faslan in Scotland. From there, he was transferred to the United States for further inquiries, and what exactly was discovered, there is still a strictly kept secret.