Okay so here's a breakdown of what's been going on. All following information is now public.
For some time now we've seen an increase in russian activity in the Baltic Sea, most of it is classified and has not warranted an operation of this magnitude. The reason why the shit hit the fan this time (so to speak) is due to a combination of both HUMINT and SIGINT. As the operation progressed more observations were made that formed a pattern and let us to believe that foreign underwater activity is or has been underway in swedish waters. As long as this pattern continues so will the operation.
What's important to know is that officially this is not deemed a submarine hunt with all that definition entails. Also no foreign power has officially been marked a suspect if you will. Due to severe downsizing we lack the proper equipment to conduct a full on submarine hunt, this is an intelligence operation.
With that said, should a submarine be located within swedish waters it will be forced to surface with any means necessary. The chances of that happening however are very low, statistically speaking and due to limited capabilities.