The obvious answer is that they were testing their own sub detection system for gaps (perhaps close in gaps)...as in, could a foreign vessel "crawl" past our sensors without being detected.
Based upon the results, I'd guess "yes."
Then you'd have to wonder if that particular area had something special that needed protecting from foreign vessels getting too close in.
It was the US that secretely tapped into their military communications cables for years in the same area. We specially modified a submarine to perform the 'tapping' operation. One of the US spies that we finally caught told them about our tap. So, a little paranoia is understandable.