I am guessing the Russian sub was there testing Swedish defenses but was allowed to depart intact.
It is a cat and mouse game. If you detect and track it, do you make it obvious you are? Then the other side learns they aren’t invisible. Or maybe you keep a loose hold on it while pretending to continue to search. That denies the other side the knowledge that you can detect them. It is all head games about what you reveal or not, confidence, etc.