Usually because the Western submarine was tailing them at the time, a fundamentally risky business. There was a collision between UK and French SSBNs on patrol

Usually because the Western submarine was tailing them at the time, a fundamentally risky business. There was a collision between UK and French SSBNs on patrol a few years back, and indication as to how quiet they can be - and SSBNs warrant the absolutely best ASW technology and techniques. An SSK, like the one missing, that has lost power isn't going to be making any sound at all unless the crew are doing it. If it is on the bottom you're essentially going to have to do a sea bed survey. The UK's Holland 1 submarine was lost under tow in 1913 only 1.5 miles outside Plymouth breakwater and remained undetected until 1981.