February 2, 2021 – The U.K. Royal Navy's Trafalgar class attack submarine HMS Talent has arrived in the British territory of Gibraltar sporting curious new sensors on either side of its sail. We can say with near certainty that these are additions to an existing system designed to detect enemy submarines without the use of sonar that first appeared on the boat in 2019.
In addition to the sensor suite, the submarine also has a pair of 7.62mm FN MAG machine guns, known as L7 General Purpose Machine Guns (GPMG) in British military service, mounted on pintles on top of the sail. The fitting of machine guns in this manner, together with arming individual sailors with small arms and non-lethal weapons, are common force protection measures employed by the crews of Royal Navy submarines, as well as those belonging to other navies, especially when entering or exiting ports.
As for what these sensors actually do, they are designed to detect other submarines beneath the waves via the changes in water density that they leave in their wake. These systems have historically been almost exclusively associated with Soviet and now Russian submarines.
In principle, such a system would be a valuable additional passive sensor for a submarine when hunting for other boats underwater.
Spanish Environmental Group wasn't happy – Gibraltar warning as 'suspicious' US nuclear submarine arrives - 'People must know' | World | News | Express.co.uk
From 2018 - A Royal Navy vessel fired a warning flare after Spanish Guardia Civil vessel got too close to nuclear submarine HMS Talent. It is understood that HMS Talent was departing Gibraltar after having her Tomahawk missile stores replenished. This is becoming increasingly common. Earlier in the month, the USS Newport News, a Los Angeles class submarine, was harassed as she visited Gibraltar. Local media reported at the time that eyewitnesses said a Spanish customs boat was intercepted by a Gibraltar Defence Police vessel after it came too close to the US submarine.
A side note: