Pasta is correct - all submarine arms spend their time trying on sneaking around. TF reminds us of the Soviet navies efforts in Sweden and of course the Finns often seem to have "unknown objects" wandering about their waters.
The British and the French managed a collision a few years back and there are stories, sometimes published as part of someone's obituary, about some scary close encounters in the past.
No-one talks about today of course but you can bet it's going on 24/7